You answered your own question by saying acft with different speeds. A safe number with all C152's and say one twin Baron ie YPJT is going to be different with a tigermoth,grumman,C172 and different twins doing asymetric go rounds.
Always a consistent problem at any major GAAP ad but other facors such as weather, ATC abiltity, adherance to ''common sense cct size'' and how things are going to go if one of th acft has an emergency. An engine failure on D'WIND with a close base and landing with 5 acft in front with inexperienced pilots could really compound the problem.
I must be bored for writing this as well...