Jan - you are "foreign", so obviously you cannot really understand!
UK air is very complicated, so a Rotax cannot possibly run without a fuel return line. Everybody knows that!
So virtually all Rotax fuel systems on UK aircraft have a return line. Given the plumbing involved it really mans that returning to just one tank, not necessarily the one you are drawing from is pretty common.
As for the initial request - well who cares? Plumb the lines so you use the fuel from the pilots side. Burn an hours worth, change tanks for half an hour and then equal times.
Realistically the return line Tee piece hole is usually around 0.1mm diameter so the return amount is pretty small, enough to overfill a full tank but, but, but,
Take the average fuel pressure and work it out, my fuel pressure seemed pretty constant, unfortunately I cannot remember at what pressure!