It hurts my brain to do it that way.

300 sheets for 1.2 is a nice division by .6: 500 sheets for $2. So now all we gotta do is multiply by .7 to get 350 sheets for 1.40.

Considering the fold, we are at 330 effective sheets for 1.40. 350 > 330, get the 3 ply (lol).

This is a little bit funky obviously and just works because of seeing a divisor. If not for that I would have tried once each way to get cost per full effective sheet or cost per half effective sheet to see which one has nicer numbers.

... Or just open the calculator app.