the ideal way of doing it now, is to have a plant ready to go into flower when your flowering plant is done. ya know one comes out a new goes in, doing it so keeps you medicated and happy. just a little rule of thumb would be about 2 weeks to root the cut, another 2 to veg out a bit then flower. in your case you may want to veg more than that just to get the height and branching up to get it started in the screen before flowering. its all about timing for the perpetual style garden. lets say you know that this strain takes 60 days to be ready to harvest, then you also know it only takes a week to root the cut, but it takes 4 weeks to get it to the size it needs to be to get it ready for the screen. so that would mean you would need to start your cut at little more than 3 weeks after your other plant started into flower. this is just an example to give you an idea for getting started with the strategy that will fit your needs.