Floetrol is a *retarder*, meaning that it makes the
paint dry more slowly. The primary benefit is that
you can keep a wet edge longer. To a first order,
Floetrol doesn't *thin* the paint.

Thinning the paint (with water) will make it level better.
If you do this, do it only slightly (5% water), as thinned
latex will not have as hard a surface (wear resistance)
as unthinned latex.

I use Floetrol only when brushing, and Floetrol plus 5% water
when spraying latex.