Anyone care to test my formula?

Sorry to tell you this but your formula must be flawed somehow. My stats are

ATT 8
STR 6
DEF 8
HP 13
PRA 8

(Magic and ranged both 1)

Your formula: combat lvl 11 (excactly, no rounding either way)
Actual combat lvl: 10

As to whether it's rounding up or rounding down... I think one possibility is that there's no rounding either way, the game just doesn't show the decimals. So in a sense it's always rounding down

the calculator on this site is as good as your gonna get for now, until someone sits down and puts the effort into making a pure with just lvl 99 prayer to figure out exactly how it's calc'd because that is the only trick to the formula.