Converting clinically relevant dose for in vitro studies - (Jan/30/2015 )

Hi, can anyone suggest how to convert clinically relevant doses for in vitro studies. For example, say the clinically relevant dose for 5FU is 400 mg/m2; how do I convert it for MTT drug response assays. Tq

Bump. I am also interested in finding out how they do it. I mean actually, not guesswork.