You can do a somewhat educated guess by bearing surface comparisons and how the load is related to the throat OAL basically.
But there are other variables and the easy way for me thats been safe, back it off a couple of grains, 1-2 rounds and load it up back to normal and run em over a chrono. One of the very few uses for the chrono IMHO.
No way would I load a lot of the same load. Nor would I waste personally more than 1 shot going back up a grain at a time basically.
I would certainly clean the barrel first, very well and fire 5 fouling shots of the low charge though before I went over the chrono with test loads
