welcome to the board!

A sleeping bag is an ambitious first project. The final weight of it would depend on a few things, such as shell material (momentum? ripstop?) and zipper (full length or partial? metal or plastic?).

I have no experience making a sleeping bag, but I have made backpacking quilts with climashield xp and also with a kit from Ray Jardine's website (ray way quilt with alpine upgrade) and found that I far prefer the insulation in the ray way quilt. The Climashield wasn't as compressible or lofty as the insulation in the Ray way (3D? the website doesn't say).

Based on my experience making the quilt, I would guess that you will need more material for an oversized mummy. Your estimate is about what I had for the quilt, and I am not a very large person.

Do you have a pattern for the bag? Are you going to sew through all the layers or use yarn to quilt the insulation in place?