Here's the answer from MSD....one degree per thousand rpm, would be less than one degree timing variation at idle, where most of us use the timing lights I would expect.
I don't think that variance would be worth buying another timing light!
Sir;
Adjustable or digital timing lights can pick up interference from the MSD and can produce inaccurate readings. As much as 1 degree per thousand RPM. We recommend using a standard analog timing light.