I think I could safely take the maximum load for a bullet that is 20 grain heavier and start there with load development.
Most likely you are right and that'll work fine, but I'd back off a bit from that, maybe another grain or two, just to be safe. It'll cost you a few extra bullets and primers but it's worth being extra cautious with starting loads if you don't have actual data. Most of the time what you describe works out great, but I've run into enough cases myself where that technique ended up being a max load or even blowing primer pockets.
To be clear, I have no problem working up to hot loads, I'm just advocating starting a little lower to be sure. Do please post your results though; I'd like to see them.
