Is there a particular reason you are after a relative metric? Why not look at absolute delay?
I mean, as you said before, the delay on a local box is super small, practically zero. If you'll try to calculate a quotient (remote delay)/(local delay), you'll get huge numbers, and very large inaccuracy.
An absolute measure of the delay would be a much more accurate one, in terms of numerical stability.
However, I believe the ping echoes are given lower priority than let's say, the data being transferred through ssh -X session, so ping need not be the same as your text editing delay, nevertheless, under normal network and processing load, it should come pretty close.