I use 25% in my sugar washes,seems to work good I have had problems when going to 50%,it has been my experience to much backset slows it down! Starting to get into mashes so dont know yet but going to start at 25%.
It depends what it is and how long you inend to run it.
I have done plenty of 100% fruit washes that get 1 repetition using 100% backset and sugar. I'd then blend these if the second wash was good enough to make the fruit go further.
Using UJSM though i use 1/4 or 25% as if i go higher with the backset it will eventually over a couple of gens drop the PH to show stopping levels and the wash will stall.