How do you determine when to flip a photoperiod clone into flower?

Multiple factors. Some strains stretch more than others. Since you're running several plants in a pretty small space, you're probably want to flip a bit on the early side. A 4' tall tent doesn't leave a lot of headroom for lights so you don't want these getting too tall.

When you do SoG with cuttings, you usually only needs 2 weeks, the time for the root system to populate the substrate. But if you want grow a full size plants, you can let your cutting in vegetative growth as long as you need.

“Cuttings” can be switched as soon as desired, however, for optimum yields it is generally best to wait until the clone has a strong root system (typically 2-3 weeks old). For “seedlings”, if space (and time) permit, better yields can be obtained from a 6-8 week veg period.