Yes, a 1/2 size setting should do the trick pretty well. On very rare occasion, you might land at the extreme edge of the CFZ, so if you wanted to short that 1/2 by a tad, that would give a bit of insurance that you're always going to be in the zone.

Seeing is often estimated by double star separations (visual observers do this regularly). You can get a coarse feel for seeing over time by watching HFR floor values during focus runs too.

Generally, I don't change my step size. I know my site, and it's nominal seeing spread. I factor the step size for good/very good conditions, leave it there, and accept that the "actual" CFZ for a lesser seeing condition will be larger. I accept some wasted measurements in those conditions (while praying for seeing to improve!) ;-)