Lets try reverse-engineering this thing a little. Centaur is probably the weakest Large creature. He has 18 Str. +6 from WH. At least 24 Str, almost 25, which is close to your estimated 2xStr in pounds. So I guess it could work. But OTOH it doesn't scale too good. One point of Str adds only 1 lb., but the higher the Str the more carrying capacity each new point gives. So maybe it would be better to make it "light load divided by 10"? Then going from 24 Str to 25 would actually give 7 more lb.
