Filing the nose down to a wadcutter shape will reduce the BC a lot. I don't have a direct way to calculate it for the exact shape of the .22 LR, but by stealing an analogy from .32 caliber, I can tell you that averaging the BC's of an 83 and a 90 grain full wadcutter gives a G1 BC for an 86.5 grain wadcutter of 0.0365. There is an H&N High Speed 130 grain round nose in .32, which is very close to 1.5 times heavier than the 86.5 grain wadcutter would be, and is round in the ogive, like a .22 RF. It has a G1 BC of 0.234. Dividing 0.234 by 0.0365 give a ratio of about 6.4:1.

That suggests an approximation for a filed flat .22 round nose at 27 grains, the RA4 BC would be about 6.4 times smaller than a 40 grain round nose. For a 40 grain bullet, If the RA4 BC is about 0.1, then a wadcutter conversion that removes a third of the weight should result in an RA4 BC of about 0.016.

If you are only going for 75% flat meplat your reduction ratio on your 29 grain CB cap will be less. I suppose a factor of 3-5 times smaller than with the round nose might be reasonable, so 1/4 of the 29 grain CB long RA4 BC would be my first estimate. But please note that a wadcutter or a flat nose will not fly with the RA4 drag function shape, so this is all a bunch of SWAG anyway. On the other hand, you are subsonic all the way, so it may be close enough.

The actual muzzle velocity would not be quite as much as you predicted if a powder were propelling the projectile because it would burn less efficiently with the lighter bullet. Primer-only mix, though, should go off right away, so your velocity difference estimate may be about right. You'll have to try it to see?