Calculating Carrying Capacity - Need Exponents

I've boiled down the carrying capacity formula from D&D 3.5, however I require the ability to use exponents, because a Strengths of 20 and higher, the values are just the 10-19 values, multiplied by 4^(floor(@{STR}/10)-1). I've looked into the CSS Wizardry thread, while there is a rudimentary pow(x,y), but it looks like calling it requires you to know both numbers at design time. I know X will be 4, however Y won't be known until run time.

@{STR} is going to be in the range [10,30]? I think the following gets the right values, but I'm not 100% on that: 9 * floor(@{STR}/10)-1 * floor(@{STR}/10)-1 / 2 - 21 * floor(@{STR}/10)-1/ 2 + 7

Creatures can have over 30 STR simply by virtue of their size. Colossal creatures get a +32 to Strength from size. I understand the point, though, and I was hoping to avoid doing that. The inability to do exponents means I'll have to apply your advice from another question I asked and catch the changed STR value and send the carrying capacity to the character sheet from the API.