Reading the negatives the same every time is not good enough imho.

I use a densitometer with the built in settings, you select what you're reading. It's an X-Rite DTP-820, and that has to be calibrated every time before usage.

'Low calibration' (a reading with nothing) which should calibrate to 0.0 0.0 0.0 etc, and a high reading (a set of values usually near 3, you enter the density values of the calibration card, so you can make your own etc).

Even if I come back an hour later, both the high and low readings will have shifted. So if you go back and read the same neg again you will get values that are a bit different.