uncurve/curve comparison on Loris' website
I don't know if this helps or not, but I made a comparison of the same image, same workflow, but the darker one is curved but not with correct curves, and the top/brighter one is custom curved for each color. Loris graciously posted it on his website, below. Now, it is not that the bottom one is bad--in fact, I sold it. But in a side by side you can tell what a curve does so it is a great teaching tool. I thought the bottom image MIGHT relate to when you say your image is "too dark" but maybe not?
I am not going to lie to you and say it didn't take me a lot of time to come up with individual curves for each color. It takes, with gum anyway, several hours of time per color. But if you want to loosey goosey it you could use a longer exposure time for yellow and a straighter line curve, and with thalo and magenta, an inverted/backwards s curve that flattens out the negative pretty considerably. I have no idea, tho, what printer you are using and all this may not apply to using all inks and a different printer than the 2400. Suffice it to say, I think my example is a clear indication that curves for gum is a justifiable method for a practitioner to use when fine tuning the gum process, but not wholly necessary when starting out (the fact that I sold the image for instance). Does that make sense?
Christina Z. Anderson
Assistant Professor, Photo Option Coordinator
Montana State University, VCB 220
Bozeman Montana 59718