Generic encounter formula

For an encounter, generically (provided the creature types involved are viable for such a thing),
I think a basic formula to follow would be: (mind you these # are somewhat arbitrary, but just sort of a starting point to work out a system of sorts to apply to most enounters)

2 Ranged Physical (CR = [avgPC level - 1] + [# of party members]) rounded down
2 Ranged Magical (CR = [avgPC level - 1] + [# of party members]) rounded down
2 Melee flankers (sneak attackers?) (CR = [avgPC level -2] + [# of party members]) rounded down
2 Melee Tanks (CR = [avgPC level -2] + [# of party members]) rounded down

A full spawn of 8 enemies, each individually close to the PCs level.
The gear on the enemies would be very generic and low power, to allow
better equipped PCs to gain an edge.

So a party of 5 level 10 PCs would end up facing a group of 2 CR14 archers, 2 CR14 casters, 2 CR13 flankers, and 2 CR13 tanks. A tough fight, but winnable with decent gear and tactics.

This could be adjusted for the gear level of the enemies of course... so better equipped enemies would have a lower initial CR and/or gain less of a bump from additional party members.

Also adjustable based on how many enemies you want to have in the spawn, less enemies means a bump in CR or gear level.

This formula also assumes that PCs will generally be better geared than the enemy, but out numbered and be slightly lower level than enemy CR.

Ideas on improvements?