R grail = 2 Rs worth per skill level -> as long as skillleveling it costs less than 2, you're getting a profit, so ideally you'll level them to either 3 or 4 here (depending on if you feel like doing the 66% gamble, which can be quite annoying/time consuming).
SR grail is worth 5 per level -> usually level it to slvl 5, where it's worth 25 Rs, SRs 3.5 -> slvl 4