There's no point in measuring something more precisely than the thing itself exists. Which pixel "is" the base of the finger?

Has anyone tried repeating the measurement, following the same procedure each time? Do this on occasions at least a day apart, to avoid unconsciously imitating the second time exactly what you did the first time. I have. Reproducibility was no better than 1mm, and pretty much independent between fingers.

You could also try under different conditions, e.g. when your hands are cold and when they're warm, when you've just been exerting them heavily and when you haven't, etc.

I doubt that digit ratio exists as an entity to better than 1 or 2% accuracy.