Perrenial Vella that I was, I used to do loads of analysis of my AIMCATs and my projected percentiles
I used the formula :-
%ile = 1- [(Rank in AIMCATs) * 3.5 / number of students scheduled to appear in CAT ]
Averaged across AIMCATS and CL Mocks, this expected percentile( E(perc x) ) came out to be 99.51, my final percentile was 99.45
Disclaimer: Don't take it as "rambaad" and come abusing later :P