I hope someone can come in and give you the right answer. When you say 80% of gross or net I think of wages as in earned income.
That would be Gross, less the medical neccessities as medication, bus fare, driver fare, equipment needs.

IF you are self employed it is off the net.
I have a structured w/c settlement of a nominal amount that does not affect my SSDI. (I received a lump payment, but the settlement was written as structured)My wages are below SGA and I have had no medical improvement.

I guess you are talking about the w/c offset to still receive some SSDI also. In my case I was earning too much w/c and it would not ahve provided for any w/c if I had not settled. I would ahve been awarded, but still only received w/c.
Have you tried to search the www.ssa.gov site?
Dianne