I ran the numbers through QuickLOAD V3.4 as well as did a quick OBT (Optimum Barrell Time) analysis of IMR 4007 SSC with a 20" barrel and came up with this,... provided your fired case capacity is 56.00gr of water (I'll change it if you want, let me know):
At Temp of 65F:
45.0gr (98.8% fill capacity, and estimated 2454fps) of IMR 4007 SSC is the optimum load for a 20" Barrel, but you'll have to play with seating depth to determine the most accuracy. PMax (pressure) for this load is only 48784psi (60191psi max for .308win), so it should be a pretty safe powder to play with.
42.5gr is also a OBT load as per QuickLOAD, but only generates 2309fps,...which is REALLY slow and might even border on dangerous (detonation),...
Keep in mind that these are simply computer models and are not based on reality i.e. your rifle, your barrell, your conditions,... so use them with caution. However, I've found that QuickLOAD is pretty spot-on in regard to the charge required to zero in on an optimum load, all I need to do usually is play with the seating depth to get it dialed in. Hope this helps!
Ryan