I do not think there is a “rule” per se. from my own experience (for BP only) I can press sum of db weight equal to the amount of weights on the ends of the bar. (E.G. 135# barbell would be 45#db’s in each hand. So try subtracting the 45 (or 20k) bar and find db’s equal to that amount of weight.
It could be just me, so start slowly, please.