You are correct in that the Income page for SSA benefits doesn't ask for the gross and net. I guess what I was assuming was that the Medicare Part B amounts as shown on SSA 1099s would be deducted from Box 5 figures to then reflect what the (net) taxable amount would be on Line 6b. So that doesn't appear to be the case and somehow IRS just comes up with its own figure which I cannot find any explanation of how that is determined.
