I dusted off my old PC and found Lee's rules of thumb for internal ballistics. He said they came from an older VihtaVouri data book (one he noted was notorious for hot loads). The two relationships are: 1) increasing the next case capacity by 10% while holding the charge constant drops peak pressure 13% and velocity 3%; and 2) increasing the charge by 10% in a given case will raise pressure 20% and velocity 8%. (Based on the latter relationship, I'll hazard a guess the data was taken with single base powders and used crushers for pressure measurement.)
So, for your problem (where you wish to use the same powder in the parent and the blown out case), blow out the case 10% and your pressure will drop to 87%. To get your pressure back, you need to increase the charge enough to raise the pressure 1.00/0.87, or a 15% rise. Your charge increase is, then, 10% * (15%/20%) = 7.5%.
Going through the math for velocity, your 10% bigger case will give you 2.8% more velocity. This compares well to Barsness' 4:1 rule, which corresponds to 2.5% more velocity.
Going to QuickLOAD (and my copy is 10 years old), it finds that single base powders for a .257 Roberts size case will need about 6.3% more charge to maintain pressure when the case is blown out 10%. Velocity will increase about 1.3% --well below the rule's 2.5%.
Rules of thumb and simulators are just that. It's hard to beat pressure tested data!
Lastly, the old Powley computer will let you quickly estimate new powder charges. In this case, it predicts you'll need 10% more charge but of a powder about 3% slower in burn rate. The velocity increase will be about 2.6%.
