Letro is a powerful AI (aromatase inhibitor), used to control estrogen (e2) while on cycle. One needs to be very cautious when dosing letro because it can easily to lower e2 too much. A 0.25mg dose, or less, of letro may inhibit 98%+ of e2. Either aromasin or arimidex would be a better choice unless one knows that they are a high aromatizer by having blood work on cycle. IMO AIs should
only be used on cycle to keep estrogen within range. Nolvadex and clomid are the best products to use for PCT.
If you haven't already you should give the attached thread a good read. Jimmy does a great job at summarizing various ancillaries and their dosages.
Ancillary Reference Guide