James Fallows' Oct. 3 column in The Post charges that senators and columnists are "pretending that the issue is whether the budget should go up by 3 or 5 percent" for defense spending. He further alleges that the "5 percenters" are reluctant to get specific about how to spend such an increase.

Mr. Fallows has missed the real issue: are we as a nation willing to do the minimum required to maintain an adequate defense?

That minimum happens to be 5 percent, and the specific areas are obvious to any informed defense observer:

1) $70 billion just to restore the administration's 3 percent Five Year Defense Plan (FYDP) due to unrealistically low inflation rate estimates.

2) $18 billion in 1980 dollars to restore underfunding of programs contained in the FYDP; $12 billion of this would go for readiness and support programs, missile stockpiles, equipment and spares, and munitions; the remaining $6 billion is needed for basic system procurement such as the XM-1 tank, the ROLAND air defense system and the Navy F18 aircraft.

3) $12 billion to begin to redress the imbalance between forces-in-being and national defense strategy over the next five years. Some Indian Ocean-Persian Gulf presence, increased airlift and sealift would begin to be provided. It will require 15 to 20 years to regain this critical balance.

4) $8 billion for strategic programs planned or ongoing but not funded or not fully funded in the FYDP. Among these programs are MX basing, ground- and sea-launched cruise missile R&D, Trident I and II missiles, Trident submarines and B52 modification programs.

5) $8 billion additional for Navy shipbuilding. An average of only 13 ships per year are in the FYDP while 17 are required to maintain current force levels. Simply to maintain a thin margin of naval superiority in the mid-1980s, 22 more ships are required.

The foregoing should provide Mr. Fallows with the answer to his first question, "How much is all this really going to cost?" These requirements dictate an increase in "real growth" of $46 billion in 1980 dollars or a net 5.5 percent real growth in the defense budget. The funds certainly do not provide for any of the new and exotic weapons alluded to in his article.

Now to address some of the other misleading assertions:

The original contract cost of the B1 in December 1970 was $40.1 million. When the B1 program was terminated by the president in June 1977, the unit cost had risen to $48.5 million in 1970 dollars (20 percent higher) or $101.7 million in inflated 1977 dollars (250 percent higher) -- not the quadrupling charged.

"The more complex the weapons system, the worse the problems of maintenance and reliability," Mr. Fallows alleges. This is not necessarily true. For example, over the last 12 months, the new "more sophisticated" F15 and F16 Air Force fighters were unflyable only 25 to 30 percent of the time compared with 40 to 50 percent for the F4s they replace. Maintenance hours per flight hour for the F15 and F4 were the same; for the F16, about one-third less. The Navy's significantly more capable F14 (which simultaneously engages six targets while tracking 18 others) has about the same record of maintainability and reliability as the F4.

Will new-generation weapons "really make a difference?" Yes. The guided TOW anti-tank missile referred to is much more advanced than Mr. Fallows' outdated understanding. He asserts that the soldier guiding the missile, because he is exposed, is virtually assured of getting hit. The TOW weapon is now deployed in an armored infantry vehicle with a periscopic, night-capable sight. Moreover, the TOW missile has a range about one kilometer more than the current effective gun range of the tank it is designed to destroy. The soldier is out of range.

Infrared systems do not require "clear daylight conditions to work." They are designed for night use in that they rely on heat emissions for sighting and guidance. Research and development will further improve the capability of infrared systems "in the real circumstances of war" such as battlefield smoke and dust as well as in inclement weather.

When outnumbered by 4-to-1 in tanks, 6-to-1 in armored vehicles and 3-to-1 in tactical aircraft, force multipliers are absolutely essential to balance the military equation. The "smart weapons" must make the difference.

"Does out spending respond to realistic threats?" I do not intend to answer in this article Mr. Fallow' oversimplified one-paragraph lesson in missile ballistics. Suffice it to say that ample expert testimony exists to establish the need for and efficacy of the MX mobile missile system. Analysis shows that ascribing a "bias" to Soviet ICBMs twice the bias estimated for our Minutemen (which ignres reasons that indicate that Soviet "bias" should be less) will still result in the destruction of 80 percent of our force instead of 90 percent. To contend that the MX responds to an unrealistic threat is simply not supportable.

The "focus" of our defense spending requirements is addressed in large measure in the beginning of this article. A 5 percent increase is more notable for what it does not contain (in the way of "complex hardware") than for what it does; none of the popularly referred to "add-ons" is included. The simple reality is that other less glamorous but higher priority requirements will not allow it. About one-third the increase called for is to address shortfalls in readiness and support programs, including training. Approximately one-half all defense procurement is "readiness" procurement as opposed to modernization procurement.

There is nothing magic about 5 percent; it only happens to be the minimum required to begin to reverse the downward trends, vis-a-vis the Soviet Union, in conventional forces, theater nuclear forces and maritime strength, as well as strategic balance.

Mr. Fallows is correct. We must continue to seek "better ways" to spend our defense dollars. Both Congress and the executive branch have indicated a commitment to do just that. Even with an increase in the defense budget, it will not be adequate without such measures. But with or without these measures, the present defense budget must be increased. It is simply inadequate.