gear geometry

can anyone suggest me what are the practical limits for gear geometry. We normally practice..

tip thickness--min 1.5mm

transsverse contact ratio--1.2min

total contact ratio--2.3min

addendum modification factor max--1

helix angle <23 for 1st gear, max 33 for rest of the gears generally

do i need to check anything else apart from undercut & life

how do i rate a gear pair for nvh performance...

and the last question...can anyone tell me whats the gear optimization algorithm useed for strenght, contact ratio, pitting etc???

thanks in advance