Which photoperiod on new setup

Just depends on how large you grow them. Growing more plants in the same tent just shortens the vege phase (up to a point - common sense required to interpret that correctly). 300w, fairly efficient light if specs are accurate... it'll be great for a 3x3 tent or even slightly larger. 840umol/s PAR production from that light, if accurate. So, on a 12/12 cycle and assuming it's shaped well enough to distribute the light, it can cover up to 1m^2 and still make full use of ambient CO2 conditions and a climate that at minimum avoids extreme conditions. Read up on DLI for how to size this stuff. All suggestions require minor adjustments for local variables. so, you can grow as many plants as feasibly can fit into 1m^2 with this light. Any canopy that isn't larger than 1m^2 is fine. This isn't a hard line in the sand... you could go a bit larger and results wouldn't suffer too much, but go to far and they will suffer eventually. Smaller will require less than 100% power on 12 hour schedule.