Well in case anybody else is interested in using these things as a dream aid instead of a full-on trip, I have decided that in accordance with the dosage thing:

Oral Diphenhydramine Dosages
(Recreational)
Threshold -- 10mg
Light -- 25-150 mg
Common -- 150-300 mg
Strong -- 250-500 mg
Heavy -- 400-700
LD50 (Lethal Dose*) -- 500 mg/kg in rats

...and based on this experience report, I'm gonna take eight pills. I'm gonna be in my room and I'm gonna stay up for two hours after I take them, but when I start to feel the trippy effects I'm gonna let myself doze off and hope they translate into dreamspace.

Erowid ho!