Yep, you're going in the right direction. A long time ago a group of us were building big and light, and we settled on 16 oz/sqft as giving a reasonable loading for scaleish flight. Was easy to work out too, wing area in sq ft equals airframe weight in pounds. There was some modification, WW1 and earlier were better around 10~12 oz/sqft, and WW2 and later looked better around 20~24 oz/sqft. We didn't expand on the envelope as you had to build lots of models to test the theory, and we just didn't build that many...
we got lots of comments on our 'helium filled' models, but since most of them are still flying, and 15 or so years later, you needn't be concerned about the strength or longevity of the things either...
Evan, WB #12.