How much taller will they grow?

Evening all,
Newbie need a little advice!
After a bad batch of autos I've managed to grow 2 girls(2nd week of flowering and they are female) from the free normal photoperiod seeds that came with the order.
They are about 28 inches tall second week into 12/12, I'm assuming they have about 8 weeks of flowering left, is that right?
How much taller will they get? I've got about 18 inch of space left !!!
If they're going to grow more than that what can I do?

Thanks

if they are indica dominant you may be safe, but pushing your luck. Sative dominant and they may still grow another 3ft or more... Just have to wait and see basically. There is no exact methodology that I know of.

They are UFO#2 Reserva Privada Cole Train and*Serious Seeds Chronic.
I had them on 24/0 and at about 12-14 inches I changed the photo period 1 hour per day until 12/12, during that time they have doubled in height.
If they grow more than 18 inches more I'll run out of space!!!
If that happens can I do a bit of LST to keep them down?
Cheers

yes, if you are careful you can gently bend them a bit and tie them that way.

well the first third of flower is the stretch period
last third buds just fatten up

but anyway stretch based on % of indica and sativa that makes up your plant
cause indica doubles at stretch and sativas can triple at stretch

and if say yo plant is 75% indica and 25% sativa it would probally (guess) double and a quarter but you get the idea - right
yes can just supercrop that beotch just squeeze and bend will grow a knuckle there and keep on keepin on buds will point up and be bigger