I think I understand what you're shooting for, but I think the important factor here is going to be your time to harvest. Remember that size can be controlled a lot of ways, LST, etc. and you can use those methods to buy yourself several inches without impacting yield if vertical space is an issue. If you know the strains you're using well enough that you can reliably predict the finishing time, then you can accurately make your own harvest schedule.
For example: if you know that your plant will finish in 8 weeks from the day you put it into 12/12, you can take the single clone from it and veg it exactly 4 weeks. (Your clone doesn't have to be 8 inches tall, it'll be vegging for a month. The clone you take could be 3-4 inches tall and it'll be fine... Buds is right though, you'll want to select a strong shoot, not a weak, spindly one as your clone candidate.)
Then, when you harvest the "donor" plants, the clones will be ready to go into 12/12 ...right after you take a single clone from those. You don't have to wait any amount of time after taking the clone before you put the 'donor' into flower. You could grow your plant to your desired height, take a cutting/clone (don't take the clone from the top meristem, use one of the side shoots), and put the 'donor' plant into flowering immediately. Ideally, 4 weeks later you will take another clone from the clone you took from the original. It would look like this:
At Day 1: Donor (A) is ready for flower, take a clone (B) and put into your veg room. Donor (A) gets put on 12/12 light cycle in the flower room.
At Day 30: Donor (A) is one month old, and you know by knowing your strain, that it is exactly 1/2 way through flowering. Plant (B) is now a rooted clone one month old and ready to have a clone taken from it. Take a clone (C) from Plant (B) and put it into your veg room. Move Plant (B) into flowering 12/12.
At Day 60: Donor (A) has been 60 days in flowering and is ready for harvest. Plant (B) is 60 days old and you know is now halfway through flower. Clone C is now 30 days old and ready to have a clone taken from it!
At this time, Day 60, from now til forever, you'll be harvesting one plant every 30 days.
If your clones are not tall enough, not to your desired 12-18 inches, you can easily adjust by holding your donor plants in veg another week or 2 after taking the clones but before flipping them to 12/12. The flower time stays the same, so your donor plant finishes in 60 days, at 30 days you put your (now 44 day old) clones into the flower room and so on. You'll get a feel pretty quickly as to how long it takes to get your clones to 12-18 inches height, you may indeed have to change your schedule so you take your clones a couple weeks prior to putting the donor into flower. If your clones are growing too tall, a little LST or even topping will bring em back down from the heavens.
With all else being equal, after the first harvest or two, you'll have a pretty accurate idea of how much you'll yield and when they'll be ready.