I would say he is a sensible neuro...he is not in your shoes and if you told him Mestinon wears off in your case in 2.5 hours then take it every 2 or 2.5 hours!
He was also cautious to set you with a daily limit: 300 mg/day, it's up to you to manage that quantity according to how you feel.
Continue to observe your condition, it's possible that the previous 2.5 hr cycle could change.
One last word, the 300 mg/day has no magic, I've been taking up to 540 mg...and I'm now down to 300 (8:30,10:30, 13:30, 5:00, 8:00 and 11:00 and no more TR.
Call your your neuro if you have difficulties meeting it!
Maurice.