Yes, the year/quarter is an acceptable source. The only other thing you could do is buy the certificate! and most of us don't have that kind of money. This is what I usually put:
England & Wales birth registration: Yeovil [district], Volume 5c, Page 43, June quarter 1903, mother's maiden name SMITH
If you have a scanned copy of a parish register page - it depends where it came from. Did you download it from a site like Ancestry or FindmyPast? in which case their Terms & Conditions forbid it (because they spent the money scanning it).