I've ran 200mgs every three days before and made great gains! Everything was very lean and primo has this amazing ability to make your muscles as hard as bricks when contracted.
I don't think only a 100 mg would do anything except retaining a bit of muscle you already have and thats a maybe! Currently I am starting a cycle of 500 mg of primo a week with 125mg of test e a week and my goals just happen to retain the muscle I have and get as humanly cut as possible. Maybe building 4-5 pounds along the way.