I've seen an exercise like this before, but just for fun, plugged some numbers into Loadbase and did some math.

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Roughly approximating a .300 RUM: BC .711 (240gr SMK), MV 2900fps ---

1000 yard level shot = 25.2 MOA drop

For shot at 20 degree angle:

Rifleman method: 1000 yards * cos(20) = 940 yards; Drop for level shot @ 940 yards = 22.9 MOA drop

Improved Rifleman method: 25.2 MOA * cos(20) = 23.7 MOA drop

Entering 20 shooting incline @ 1000 yards directly into LoadBase = 23.5 MOA drop

"Error" of Rifleman method vs. LoadBase direct incline entry = 2.6% (~6" @ 1000 yards)

"Error" of Improved Rifleman method vs. LoadBase direct incline entry = .9% (~2" @ 1000 yards)

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Of course, the 240gr SMK is a nice slippery bullet, and the projectile is still going almost 1700 fps @ 1000.

The RUM is pretty forgiving at "only" 1000 yards though... let's try a .308 Win:

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Roughly approximating a .308 Win: BC .505 (175gr SMK), MV 2700fps ---

1000 yard level shot = 37.1 MOA drop

For shot at 20 degree angle:

Rifleman method: 1000 yards * cos(20) = 940 yards; Drop for level shot @ 940 yards = 33.1 MOA drop

Improved Rifleman method: 37.1 MOA * cos(20) = 34.9 MOA drop

Entering 20 shooting incline @ 1000 yards directly into LoadBase = 34.6 MOA drop

"Error" of Rifleman method vs. LoadBase direct incline entry = 4.5% (~15" @ 1000 yards)

"Error" of Improved Rifleman method vs. LoadBase direct incline entry = .9% (~3" @ 1000 yards)

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I'll leave the conclusion drawing to the individual reader, 'cause I'm tired