Gary Justus knows the angles.

Knowledge of a golf ball's bounces against the side boards and a precision putting stroke are the keys to the Rockville youth's prowess in the growing sport of miniature golf: Putt putt.

Today, Justus shoots for his fourth straight Rockville city championship at the Rockville Putt-putt course, starting at 10 a.m.

Justus, a member of the Professional Putters Association, has bagged bigger game. Last summer, he was a runner-up and won $5,000 in the World Putting Championship in Columbus Ohio. He earned more than $6,800 for the year.

"I put most of the money in the bank and bought a 1968 Mustang," said Justus, 17, a recent Good Counsel graduate who will attend Montgomery College-Rockville this year.

Justus holds records at two of the three 18-hole courses at Rockville Putt-putt. On course No. 2, he used 22 strokes for 18 holes, with 14 holes in one On course No. 3, he registered 23, with 13 aces.

"A lot of players just get up there and hit the ball," Justus said. "But the good players on the PPA tour concentrate more and kinda think the ball into the hole."

Last summer, Justus practiced almost every day for four hours. "I practice with about five balls on each hole. I play each hole until I find a good place to hit it."

On most holes in Putt-putt (all par two), you must carom the golf ball off the boards that line the sides of the hole. Only on a few holes can you hit the ball on a straight line into the cup.

"I just try to hit it square so it gets the same kick off the rail every time," said Justus.

This summer, Justus is concentrating more on conventional golf because he helps keep the golf carts in order at the RedGate course in Rockville. Also, his regular golf game is not too shabby. He qualified for the recent District Junior but lost to eventual winner Tom Loveless in the first round. Earlier this week, he shot 74 in qualifying for the Montgomery County Sentinel tournament, one stroke behind medalist Clay Fitzgerald.

Does Justus's magic Putt-Putt stroke carry over to the grass greens? Surprisingly, Justus uses a different stance and grip for each game.

"I try not to mix the two. It could mess one of them up. In Putt-Putt, there is more precision. When I three-putt in regular golf, it's because the speed is messed up. I guess I expect the ball to hit a rail or something."

And what does Justus win if he prevails as Rockville's best putter for the fourth consecutive year?

A gift certificate for more Putt-putt.

A record field of 117 will shoot for seven spots in U.S. Amateur qualifying Tuesday at Columbia. The amateur is slated Aug. 29-Sept. 3 at Plainfield (N.J.) Country Club.

Webb Heintzelman of Whitman High and Washington Country Club, current Maryland State high school golf champion, leaves Wednesday to represent the Mid-Atlantic section in the third annual PGA National Junior championship at Calloway Gardens, Pine Mountain, Ga., Aug. 17-20.

He earned the trip with 76-75 - 151 at Chantilly National.

Sally Voss of Bethesda, reigning Maryland Women's Amateur and Women's District champion, joins Liza Abood of Bethesda and Middle Atlantic titlist Nancy Maunder of Crofton in the U.S. Women's Amateur tomorrow at Plymouth Meeting, Pa.

A pet peeve of many golfers these hot sultry days is inadequate drinking water on many area courses. Needwood and RedGate in Montgomery County are two offenders, with tepid water available only after long stretches of holes. When they finally reach an oasis in this situation, thirsty golfers tend to become waterlogged.

Entries remain open for the Northampton Junior Invitational, Aug. 16, and the Northampton Fall Classic, Aug. 20 For information, call 336-7771.