For rosin I like to stick around 20% of the equivalent flower dosage, by weight.
For a quarter gram session I would start with a .05g dose.
I prefer hash rosin for psychedelic effects, but that could just be due to my tolerances. That said, the strongest effects I've gotten from vaporizing have been from hash rosin. Still nothing like a strong edible
If you want to improve flavor without spending much money I would definitely suggest a 20mm Quartz banger + bubble cap. This will fit your existing 20mm SH coil.
I was hoping someone would have better answers than mine!