The 80/20 rule applies. - 80 percent of the benefit is obtained by the first 20 percent of the effort. Calculating the path loss through my Nikon enlarging lens is probably a lot less important than knowing how long to deveolp to contract 9 stops to 7 stops for a particular combination. I guess what keeps me thinking about getting a densitometer is I that would like to have the whole range of the film and not worry about important things landing on the shoulder and toe. Tha requires I know the film speed for that contraction or expansion of contrast with its corresponding development time. So I can meter a zone one and zone 2 and then see if it prints. Not very scientific but it is done all the time. I still like the idea of measuring a zone 1 on the negative and getting my exposure that much closer. I have to admit though. This increased degree of accuracy would probably do little to improve my negatives because anything I care about is going to be zone 4 or 3 at the minimum and with the restraining developers I use, both Catechol and Gallol, I have not been loosing anything in the highlights either. Like using a light meter, a densitometer could help I think, if used properly - how much , I am not sure and I am not sure if the time and money would be better directed to other areas to improve my craft.
I forget about alt process people...sorry. My comments are directed mostly at conventional enlarged silver printing.
My comment about adding 10% is to point out that the image stain is proportional to the silver image, so that for a given process, there is some factor that can be added to make a simple densitometer work w/o a blue or UV channel.
For you alt process folks, I will try to run a PMK negative through the spectrometer soon. What is you spectrum of interest?