i have always used treble the altitude converted to distance...
so ...FL100 X 3 = 30 NM.works fine for me.....to decide you may on occasion ( if you know you are to be vectored around the airport for an approach at the other side ) have to ask radar to give you descent at '' 30 track miles '' instead...if you want to be accurate...otherwise the worst that happens is you get to clearance altitude a little further out...
was a very good article done on prune ( not sure which forum...maybe this one... back in december 06 on '' rules of thumb ''. maybe a search will reveal....
hope thats of some help...
the dean.
ps sorry...i see its uncontrolled..
well the same applies from your position to the fix...