I like to go with Cory's physical initiative model, only slightly altered: (Qu+Ag+In)/3 then add +2 per 5 (full) ranks in their highest weapon skill (yes, you can substitute ranks in highest spell list with combat spells, such as firebolt, lightning bolt, etc... for spellcasters). This represents the best, most realistic aspects. Quickness (duh), Agility for how well you move, Intuition for that "6th sense"/natural instinct aspect and the skill ranks because the more skilled/experienced one is, the faster they are to react in combat. Instead of the number of ranks, just go with +2 per 5 (full) levels if you don't want to count ranks.