75 Ohm BNC cable

why do you need to cut original cable? additional 15inch cable will not hurt your clock or other digital signal much.. if you  are DIY person, there are very good cables to terminate by yourself w/ BNC...

Rule of thumb says that cable becomes transmission line (reflections) when signal propagation one way is longer than 1/8 of signal transition.  Since typical transition is in order of 25ns it is about 3ns propagation.  Signal travels about (dielectric dependent) 5ns/m  making it max length of 0.6m - roughly 24 inches.  It includes connections inside of electronics on both ends, so it would be safe to go with less than 12 inches to avoid reflections.   It is a guess, since signal transitions in high end transport can be as short as 5ns while propagation depends on dielectric.  Perhaps you can use BNC coupler to reverse it, if necessary.

You shouldn't shorten your digital cable. I read that the optimum length is 1.5 meters. As the cable gets shorter reflections can become a problem. I am using two different lengths: 1 meter and 1.5 meter. I started with the 1.5 but had to switch to the 1 meter when I connected my DVD player to the TV. So far I hear no difference when using the 1m for the DAC. BTW Im is 39.37 inches.