For what level - GCSE, AS or A2?
For AS and A2, the marks for each paper are given in UMS and then added up to give a total. This differs from the raw score ie 60/70 on a paper, as it is then converted by a mark scheme to the UMS applicable - which is affected by the denominator value of the paper ( eg paper 1 might be worth 60/300 UMS but paper two maybe 120/300 in weighting. Either or neither paper may be marked out of 100, or 70 or whatever)
So if you get 90% on a paper you can't know in advance how many UMS that is - it might be 100 or it might be 80 say. One thing you do know on AS and A2 papers is that if its within a certain band of UMS you've got that grade ( eg 80 +UMS equals an A). That doesn't hold at GCSE I don't think.
I'm not sure that helps. I bet by the time I've posted this someone has added a link.......