I have to say that I have never seen a fuel sending unit in the E30 go bad. Lift pumps yes.
The sending unit is a simple and reliable design. A float moves a wiper on a potentiometer which translates to the needle swing on the guage. At approx 1.5 gal US a switch closes and illuminates the low warning light.
The OBC is pretty accurate if it is getting reliable inputs. It does not guess.