I remember having similiar problem once. I haven't done RGSS scripting in a long time, so I can't say anything about your syntax, etc. But I guess the main problem is that s1 and s2 are integers and when you divide them, they are rounded to zero before multiplied by 100 and thus the result is always 0. I think you need to convert them to floating point values before dividing them.
My explanation kinda sucked, but I hope you get the general idea...