One way to avoid getting a darker spot than desired is to have a piece of the same paper with no image (processed and toned if toning was done) using a sable 00000 brush and working the ink on to that paper until it matches the tone for the spotting then apply to the actual print. Spotting inks can be mixed to get the correct hue. Use magnifying loupes. It can be a slow process but with some practice you will get excellent results.