ON THE OPPOSITE page today, William F. Buckley Jr. expresses the view that, in defending the cause of working welfare mothers, we are among those "who write just a little better than they think." We have to note that Mr. Buckley--an exceptional writer--writes a good deal better than he counts.

Mr. Buckley notes that the Sioux City family we described in an editorial last week had a yearly income--including earnings and welfare aid--of $15,876. Helping people climb to income levels like that, he argues, amounts to trying to assure that no American has a below-average income--an obviously fruitless pursuit.

Mr. Buckley's logic is OK, but his facts are not. Of course you can't raise everyone below the average to that level unless you're also willing to pull everyone above the average down to it. But the woman from Sioux City is so far from being "close to the center of middle-income life" that the theoretical limitation is irrelevant. It is quite possible to give her a helping hand without becoming involved in the pursuit of the unattainable.

Has Mr. Buckley noticed that in recent years both prices and incomes have risen rapidly? The median income for an American family is now over $23,000. Almost all families close to this level have employer-subsidized medical insurance, which is not counted in their income, so it's quite fair to exclude Medicaid in making comparisons with the circumstances of welfare families. The average family, moreover, doesn't have to deal with the dwindling number of health care providers willing to accept Medicaid assignments.

In fact, with her five children, the woman from Sioux City had a before-tax income only about $3,400 more than the 1981 poverty level for a family of six. Without welfare help her family would fall short of that mark by over $2,000. It doesn't strike us as an insupportable drain on the Treasury--or an assault on logic--to allow a family with two working members to enjoy a living standard only $65 a week above the subsistence level, especially when they must use a good part of that additional money to pay taxes and work expenses.

Mr. Buckley says that the answer to the welfare work incentive problem is not to supplement incomes of the working poor but to make the non- working poor still poorer. Iowa is a relatively generous state, but has he thought what it would be like to raise a family of four on the $148-a-month welfare benefit that Tennessee and Alabama paid last year? How about $140 a month in oil-rich Texas or $213 in Louisiana? We think Mr. Buckley has some tacit premises of his own to review.