Our calculation factors in many things other than just difficulty/reward/rate at the time.. we factor in our pools luck on coins (conducive to coinchoose's stale factoring).. we factor in average exchange rates over time, along with average difficulty, and (hopefully) find the sweetspot of when it's a good time to mine a coin. We constantly adjust the weighting of coins.

SPT has been decent at times, but it has a rather quick difficulty re-target, and it often causes us to switch quickly which takes away hashtime that could be spent on a more profitable coin