High dose verapamil (more than 240mg a day) for migraine? Is this ever employed? My migraines aren't controlled well enough at 240mg.

Verapamil. You might ask your Doc if a trial of a different calcium blocker like amlodipine might be worthwhile. Sometimes a combination of beta blocker and calcium blocker helps also. Best to discuss with your doc.
Very common. The dose should be made on an individual basis, weighing the possible benefit of a higher dose with possible side effect risks. Giving someone more than a maximum of 480 mg per day is not recommended for any type of migraine.
Migraine prevention. Yes, you can try higher doses but we need to monitor your blood pressure & heart rate (don't want either too low are a result of verapamil). And how are your bowels moving? Constipation is common side effect. Have you & your doc considered other options to prevent migraines? 3yrs ago, AAN published prevention guidelines http://alvinblin.blogspot.com/2012/04/evidence-based-migraine-prevention.html.