I have a loaded Springfield M1A with Smith enterprises gen 3 scope mount and a Bushnell mildot legend.  I shoot off a ruck.  I have never had the chance to shoot beyond 100 yd and will FINALLY get to do so tomorrow.
I am working off of the presumption that

+2 MOA @ 200y +3 MOA @ 300y (all come-ups are cumulative, i.e. in order to get to 300 yards you would dial in a total of +5 MOA)

I am shooting Winchester 147 grain (usa3081) as it is the only reasonably priced ammo in my area.

Any advice/input on holdover, how to effective use the mildots, ect?

thanks.

That's about as good and practical advice as you're going to get.  Cutting it any thinner than that and you'd need to provide MV and BC plus altitude, temperature and humidity.

Have a good time!

good luck and nice looking rifle!

You just have to do it and see.  If you were shooting at 600 I'd say come up 12 moa from your 200 yard zero.  But you don't have a 200 yard zero yet.  Unless you zero at 25 meters  Take notes.  

Have fun, safety first and make every shot count.

For those of you using either a Garand or an M1A, your trajectory studies are pretty simple. Just memorize the following numerical combination: 3, 3, 4, 4, 5, 5, 6, 8, 8.

What’s that?

Those are your rear sight’s “come-ups” out to 1000 yards using standard M2 150 grain FMJ ammo for your .30-06 Garand and the standard M80 147 grain FMJ in 7.62x51/.308. Learn those come-ups cold, and you’ll be on target all the way out to 1000 yards, as long as you know (1) your rifle’s initial sight settings and (2) the range to your target.

How?

Simplicity itself. Assume you have a 300 yard zero on your rifle and you know your target is at 400 yards. Just rotate the left dial on your rear sights three clicks up (rotating the dial back towards you, or, in other words, clockwise), and fire your shot by the numbers.

Bang, hit, next target. Easy as pie.

Note from Cabinboy: It helped me to memorize the come-ups with the associated ranges, like this:

100-200 3
200-300 3
300-400 4
400-500 4
500-600 5
600-700 5
700-800 6
800-900 8
900-1000 8

RWVA Blog

ETA: My mistake, I see you have a scope, not iron sights.