When to flower a mother plant?

I was just wondering when to flower my mother plant? Shes only 15inchs but you guys already know how much they stretch when flowered. My tallest plant in flower is 30inchs and I'de like her to get no bigger than that. I have already took the desired amount of clones and I don't want her to get huge, and then have to trash her cuz shes to big to flower. Make sense? The clones were taken a week ago and still lookin green and lively so I think its a safe bet I'll get at least 2 ta root. So I was thinking about putting her in 12/12 tonight, and making 1 of the clones my new mother plant. Good idea?

Just flowered mine at 18inch now they are 3 foot and still growing :]

Sounds good to me, i will have the same issue once mine are in and what you have there is what i was planning to do as well.

Yeah, I'm gonna take 2 more cuttings 2night then shes going in. Shes actually 20inchs now, grew 5inchs in 2-3 days! Hopefully she doesn't get to big cuz I only have a 400watter. Its a hawaiian strain brw. Looks like a good mix of sativa/indica, mostly indica. Thanks for the replies guys.