People have vastly different tolerances with these drugs
, and tolerance develops fast, so dosage is rather hard to calculate. Also, DPH and DMH are very random drugs--that is, 300mg may be crazy one day, and 600mg may do almost nothing another day. I have heard that if one is having a large amount of allergies, those may "use up" some of the drug
lessening its psychoactive
As for the difference, dimenhydrinate is basically diphenhydramine
with another molecule attached. Because of this, dimenhydrinate doses will be roughly twice that of diphenhydramine doses. I'd recommend just using DPH to keep it simple.