Yes I know what max weight of non-lifting parts is.

If you can fly at all, you can fly at max gross.

I'm just confused by Bob Gibbons post (See quote below)

For his weight the margin on non-lifting limit is the same with or without
water the 28gals is irrelevant.

PF

Quote

-- Max gross with or without the engine is 500kg. The issue with the
engine in place is the 255kg non-lifting limit. With the engine in
place, the pilot weight determines whether the max gross of 500kg or
the non-lifting limit of 255kg kicks in first. For my weight and 28
gal of water, I hit max gross just before I hit the non-lifting limit.

End Quote