I don't get it... the maths of interchange isn't hard, if we have a standard bench of two middles, one edge, and one utility.
Five middles (three starting, two bench) each need to play an average of 48mins. This could be in the range of say 40-55mins, where the coach wants to give more mins to some players than others. If a middle isn't up to doing a 40min stint, then they simply shouldn't be named on the bench until they are fit.
Three edges (two starting, one bench) each need to play an average of 53mins. This can be in the ratio of say 65 : 65 : 30 minutes if a coach desires - but the point being each edge forward gets a breather.
Two dummy halves/utilities (one starting, one bench), in the ratio of 55 : 25 minutes (preferred player to back up player) is pretty standard.
Then we just have to be careful of some combos on the field at the same time in defence, e.g. who else is on when Paulo is there these days.