If you already started the cycle, then stick with your plan. Save the masteron for next time. What kind of ester did your friend give you? I'm guessing it's propionate. 100mg to me seems like a waste. I'd pin something easy to meter. Like 210mg per week (.7 ml 3x a week if it's 100mg/ml). You don't need super high dosages of masteron though. And I wouldn't take it for more than 8 weeks.
And this is just general info. I'm not telling you to take it. Hell, I don't even know how this would fit in with your plan (if it'll fit in with it). So don't take this more than face value.