Though there are average numbers you can use in the beginning, which most neurologists will, mestinon dosage is very personal.

It depends on a whole bunch of things, what you eat, drink, other meds, your age, gender, weight, metabolism, etcetera. I think mestinon has this more compared to other drugs, probably because it works so fast and short?

Anyway, my friend who has MG is better off with 3 times 120 mg. I on the other hand, rather take it more often and less, so around 7 times a day, +/- 90 mg a take.
It can even change in time, I used to take way less because more simply gave me side effects without extra strength. I took 3x 60 and more gave signs of cholinestic issues, which cause weakness too.
And, I used to take it on set time ("every 3 hours"), but now those years later I've learned to take it when I feel the last dose is wearing out.

Always keep in mind too much mestinon (which again is a very personal dosage) can cause weakness too.

I hope the 5 a day works for you!