Mommyof2, the amount of medicine you can take without overdosing is going to depend upon your own particular case of MG, and may actually vary depending on whether you are having a very bad, or very good day.
The advice I was given by my neuro, when i first started on Mestinon, was to increase my dosage by 1/4 of a tablet at a time. That is, the first day I took 1/4 pill every 4 hours. The second day, 1/2 pill. The third day, 3/4 pill. He said that when I noticed signs of overdose, to stop increasing and drop back down 1/4 pill, and that would be my optimum dose. You may be able to try a similar process. Add 1/4 tablet to the 2 pills you are taking, and see if you get symptoms of overdose.
With me, overdose almost always starts with muscle twitches, especially around the eyes. I may get stomach cramps, diarrhea, excess saliva. If you are worried about overdose symptoms, you might ask your doctor to prescribe the antidote to keep on hand just in case. What I used to be given was Atropine, as I remember, but now it is Donnatal, which is Atropine combined with some other meds. I may only take it once a year, but it is nice to have on hand if you ever do overdose.
If you get overdose symptoms with what you feel is your optimum dose, try taking the Mestinon with food.
Hugs,