Papa Bear told you right.... "When you fail to make PF" ....And the only way to know that is to load 10 or so rounds at the new OAL at each incremental level and shoot them in your gun across a chrono and/or read the pressure signs on the case.
There are simply too many variables involved (barrel dia, bullet dia, bullet hardness, powder batch, primer, type of cartridge case, OAL, etc, etc) for anyone here to give you an exact answer. Someone may be shooting something similar, so they can make a "SWAG", but your results will always vary. Friends here and a reloading manual can get you safely into the general area, but the last 100 yds is a trip you have to make by yourself.
It's sort of like baking cookies. You start with the general recipe, but then you make it your own by adding nuts, cinnamon, raisins, brown sugar, dried cranberries, ...whatever it is that finally tunes the cookie to your taste. The recipe can't tell you what your taste buds love, and so it is with reloading.
Hope this helps! ;)thank you