A note about calibrating the yellow layer of a tricolor gum
Loris has the right idea here. I've been taken aback sometimes to find people assuming that each layer of a tricolor gum should be able to stand on its own as a print, and wanting to calibrate each layer so that it looks good as a standalone print. No, the layers of a tricolor gum print should not look good as standalone prints (even the cyan layer, although it's likely to look more like a standalone print than the other two because it contains most of the print density); they should each look incomplete in themselves. It is only the three printings put together that make a standalone print. (This of course applies only to tricolor prints made with color separations; a tricolor print made from one negative printed three times will obviously work differently). Even when the color separations are used to print a monochrome, as I've sometimes done, you wouldn't just use one of the separations, because it wouldn't be tonally complete; you need them all together to provide all the tonality as well as all of the colors in the case of a tricolor print.