The MG Manual has a great section on Mestinon:

No fixed dosage schedule suits all patients. The need for cholinesterase inhibitors varies from day to day and during the same day. Different muscles respond differently—with any dose, some muscles get stronger, others do not change and still others become weaker.

Myasthenia Gravis: A Manual for the Health Care Provider (PDF) - page 17

The Mestinon only treats symptoms. One should take the minimum amount necessary to achieve a relative symptom free experience. It may take a bit of trial and error to get the best Mestinon schedule.

-Mark-