I use this formula for two+ boxing characters all the time. Take the lower level character and multiply 1.5 and thier level and round the result. For example a level 50 character would still get experience with a level 75 character (1.5 x 50 = 75).
If you want to know the lower limit you can group with you would take your level and divide by 1.5 (75 / 1.5 = 50).
Simple
