I've found that in instances where my ability is 7 and motivation is 34 but opportunity is something like, say, 23, I've had better performance than when my ability is 22 and my motivation is 12 compared with a 13 opportunity.
This is to say that my experience confirms that there probably is some completely subjective formula that will fool people into thinking there is some reason behind my good luck. :-)
The actual formula that academics use is Performance = AMO, ability * motivation * opportunity. I think there are a couple of modified versions but I can't remember what they are off the top of my head.