Space filler construction for a down vest

I'm hoping somebody can assist me with sewing advice on sizing.
I'm not financial at BPL anymore and I can't find the thread when Googling.
I want to make my new vest using an inner and outer shell but with the down constrained inside the shell in sewn though mesh constraint channels.
In general I know I do this by making one side [ the inner layer] the size I need and increasing the outer mesh layer by a dimension based on the loft of the finished vest.
Some guidance is needed on how much extra fabric is needed; which I can guestimate based on the finished loft.
So the question really is how much loft can I expect from 150 grams of down when made into an XXXL vest and what would be the best spacing for the seams?
I'm not financial at BPL anymore and I can't find the thread when Googling.
I want to make my new vest using an inner and outer shell but with the down constrained inside the shell in sewn though mesh constraint channels.
In general I know I do this by making one side [ the inner layer] the size I need and increasing the outer mesh layer by a dimension based on the loft of the finished vest.
Some guidance is needed on how much extra fabric is needed; which I can guestimate based on the finished loft.
So the question really is how much loft can I expect from 150 grams of down when made into an XXXL vest and what would be the best spacing for the seams?