I don't have the expertise neccessary to accurately answer your question. However, I can show you what 5 plants (from SEED) look like in soil with approximately
21 days of veg time before switching to 12/12. If I'm not mistaken (and I may be), most SOGers do not veg clones this long.
The plants below occupy a space that is 2' x 3'. This is ONLY the space the plants occupy; this does not include fan(s), or any of the other neccessities that hang out in the grow area. The first pic is the last day of 18/6 and first day of 12/12 depending on how you look at it. The second pic is appx 48 hrs after switching to 12/12. You can clearly see it may get a little crowded in there before it's all over.
Approximately 3 large clones were taken from each plant, and each plant was topped or FIM'd at 5 - 6 nodes. Both of these techniques may lend the plant to becoming more bushy. FWIW, two are indica dominant; three are sativa dominant. I'm not trying to hijack your thread, just wanted to show you what 1 plant / squarefoot looks like with 21 days of veg. IMHO, it would be damn near unmanagable vegging for 25 days with 1 plant/sf.