I know but just say average to good stuff 50mg
I wouldnt recommend you use seroquel at all especially given diphenhydramine is a antihistamine like seroquel with the mACh antagonism without the activity at dopamine and other sites.
However, if I was to use seroquel id use 25-50 mg as the h1 antagonism has a much more potent binding then its d2 antagonism which may be one of the main receptors that mediate its theraputic effects in bipolar and schizophrenia therefore requiring much higher doses.
One or two tablets should do it.