So for veg, when do you start? At 6 nodes? Later?
Earlier!
Currently I've a 11w Reptisun UVB CFL (10/30% UVB/A, see 150w light in my signature) in my 3sft veg area and start using it from the beginning of week 2 (when the seedlings are transplanted into their first pots). But it has pretty poor intensity and coverage and not all of them get the same amount of UVB. Pretty sure there is almost no UVB with more than 1ft distance. (New 24w ballast and bulb are on my list)
Rooted cuttings I put in my 8sft flowering area get 39w (12% UVB, 3ft bulb) from the first day. Normally I start with 2ft and slowly reduced to 1ft distance. This way I've not seen any stress symtomes till now.
I think, I'll use two 24w Arcadia bulbs in the future to harden them up and then switch to the stronger 24w pureUV bulbs. Such a reptile bulb has ~150-200μW/cm² at 1ft depending on the reflector and when I switch the bulbs and start again with 2ft distance(170μW/cm² with pureUV) the plants should get almost the same intensity.
Actually, this should allow a trouble- and stress-free hardening process and use of the pureUV bulb.