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.

@jasonbourne71    Reflection coming from the end of the cable (if any) can deform original edge affecting timing.  For 1.5m cable reflection from the end of the cable comes back in 15ns  (5ns/m x 1.5m x 2).  Even the very beginning of transition (knee) will come back later than 12.5ns  (middle of typical 25ns transition), where most likely level change is recognized (threshold),  It is all guessing game, since transition time and propagation time in cable can vary.  I would go even for 2m, but if you are already short (29 inches)  cutting it very short might reduce reflections.
There are also chances that characteristic impedance is matched perfectly and there is no reflection at all.

The cable is constructed in such a manner that any modifications you carry out will compromise the build and suffer a loss in audio quality.

"It includes connections inside of electronics on both ends, so it would be safe to go with less than 12 inches to avoid reflections. “ 

Circuits, Tx/Rx, I designed, match driver and receiver impedances to cable, thus no reflections on both ends, length of cable becomes irrelevant..

"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..."

Because the original cable only 0.7 m and longer but Chord Mscaler and TT2 are stacked together so I don't really need long cables.

Thank you everyone.  I think I will just order 0.7m without cutting it.