I did not flip it around.

He uses:

Y = % split-ticket voters - % straight-ticket voters

I say:

Y = -X + % split-ticket voters; where X is % straight-ticket voters.

Those two are the same thing.

A - B is the same as (-B + A), no?

I changed the order to get it to y=mx+b form and make it easier for the reader to make the leap. It's still the same thing.