I prefer to use controlled sources to take differential
voltages and make single ended, ground referred ones
that are nice and neat for whatever calculator, waveform
viewer or scripting language you may have. Plain SPICE
may not have such nice postprocessing, but if you can
make use of a cross() function, then things get real easy.
Find point of output crossing zero differential, read input
difference at that point, badda-bing.
But the details come down to dialect / software version.
If you don't have a decent results browser / viewer /
calculator, you might look at dumping raw data vectors
and parsing them for output cross, input value outside
the simulator.