Morning Beretta Shooter,
39.3 grains would give a predicted pressure of just 39,700psi, for me that's a bit too low! Too low a pressure can give unreliable results.
41.2grains gives predicted pressure of 45,400psi, that's where I'd be starting. As a "rule of thumb" starting to develop a load I go for an initial predicted pressure of about 3/4 of SAAMI Max.
With 243 @ SAAMI 60,000psi max, then 45,000psi would be my initial starting point. Likewise 223 @ SAAMI 55,000psi max, then 41,500psi would be initial starting point.
If I do not encounter Over Pressure signs, then I stop increasing load when at 10% below max PSI pressure, i.e. 54,000psi and 49,500psi respectively on the above the examples. I believe this 10% buffer
gives me a margin of safety that is acceptable; other's opinions may differ.
Maximum bullet velocity FPS, and best down range MOA accuracy are rarely found within one load; usually two slightly different loads are needed, all depends on your personal requirements, hunting or target shooting?
NB. Do not forget about the JB MILLER gyroscopic stability factor -
JBM - Calculations - Stability - If a bullet is tumbling there is no consistent accuracy!
Regards
RoyR