AS a rough possibility, divide the score by 990 (the maximum), which will give a number 0.something. Forget the 0. and you've got a percentage (near enough)- divide by 5 and you've got a mark out of twenty.
If you try it with 600 (about 60%):
600/990 = 0.606060606061
0.606060606061/5 = 0.121212121212. Grab the first two digits- 12/20
(Use the next digit or two for half marks, etc.)
(12/20 = 60%.)
I don't know if there are better ways, but this one seems to do the job.