There are many different ways to find a theoretical 1 Rep Max.  Honestly the best is to actually lift the bar, but if you haven’t done that in a long time what we will do is the following:

Using Jim Wendler’s calculation that he uses for 5/3/1 for theoretical 1 RM do the following:

RepsDone x WeightUsed x 0.0333 = Result

Result + WeightUsed = Theoretical 1 Rep Max

EXAMPLE:  5 Reps x 320 Lbs. x 0.0333 = 53.28 <-THEN-> 320 Lbs. + 53.28 = 373.28

Please keep in mind that if you have been working on 3 x 5 diligently and you have reached the point where 3 x 5 is getting difficult then its a good starting place.