When to split and transplant Hosta's?

I live in the same zone as you and anytime after they flower is fine to separate them. Hostas can be divided any time of the year, but August is best, because at this time of the year plants are actively growing new roots. I found it best to try to leave some dirt around the roots. The roots are hardy, but the smaller roots will benefit from leaving the bit of dirtball on them. (Less traumatic for the roots.) Other than that, they are crazy easy to divide.

When To Transplant Hosta

These will depend on your climate region, either you experience hot temperatures or cold temperatures but what I can say plant them in late winter. split them the time you want to transplant it do not let them dry.

any time during the growing season, if they're about to flower you should leave them til theyre finished, any time except in extreme cold

The best time to split or move any perennial plant is when it is dormant...late fall or early spring.