I trust d9's opinions here. id start out with the larger air gap, so u don't over water right away. if they seem like they can drink more u can always reduce it, or increase ur watering length or frequency.
d9's right about the relation between the pwt n the 3" drains vs the 3.5 gal buckets. but I run plenty of them with success. just pulled down a 350g plant with 300w of flower light, with the 3" drain in the 3.5's, so id say u can dial it in.
350gr must be dry, wet would be easy but that is very impressive, MM.