Actually, all available evidence points to the fact that legalising drugs like heroin actually decreases their usage. Would you suddenly take up heroin if it was legal? Do you know anyone who would?
This may be one of the scales you're looking for, btw:
http://en.wikipedia.org/wiki/File:Rational_scale_to_assess_t...
Also, worth pointing out, with respect to "amount of usage", that drugs like Ecstasy see millions of users every weekend... and yet only a handful of deaths per year (generally involving multiple drugs and/or several-days-long binges). A pretty sterling safety record, I'd say.