[Archie]

but I was never taught (Marine Corps) to do it that way.

Typical elevation for 7.62 ball ammo at 200 yards is 6 - 8 clicks of elevation. Normally another 3 up for 300 yards and then 5 - 6 up from that for 600 yards.

All that having been said, the "battle sight zero" for an M14 (or M1A in your case) with 7.62 ball ammo (147-150 grain bullet at 2800 f/s) is the sight setting for 300 yard rapid fire MINUS two clicks of elevation.

Having only a 100 yard range you might try this:
Look up on ballistic charts what the 100 yard impact is for a 300 yard zero.
Adjust your sights to hit that point at 100 yards from a slinged, prone position.
Drop two clicks and write that down.

That setting should enable you to hit a rectangle roughly the size of a human torso out to nearly 400 yards without changing sight settings; you aim at the bottom of the rectangle out to 250 yards and the top of the rectangle from 250 to 400.

A military shooter's manual for the M14 will give you good information on this proceedure also. But it will presume you have a 500 yard range.