I have written a paper on the effect of change in mass on the drag coefficient of a ping pong ball. While making my last calculations, I decided to use the slope of the final graph (where the $x$ axis is the mass condition and the $y$ axis is the drag coefficient) to do error calculations. When I turned my paper in, my teacher asked for me to declare the unit for the slopes' of the maximum and minimum worst lines of best fit.
My problem is that I don't really know what the unit would be as drag coefficient is dimensionless. I have thought about using an arbitrary unit, but any help would be appreciated.