measure your plant from the dirt or medium to top of plant,,can not count the pot,,,,for indica ,,plant,,,say you had a 12 inch plant when you start to flower him,,,you would take that 12 height and times it buy 1.5,,,,,,,,,,,,,,so 12 x 1.5= 18 inches tall when done flowering,,,,,if the plant was 2 foot tall when you start to flower would be 24 x 1.5 = 36 inches when finished,,,,,, you will find this math will tell you very close to where your plant will finish,,,if you are growing sativa it grows tall even after it has started to flower,,for sativa if you start to flower at 24 inches it would be 24 x 2.5 = 60 inches so it would be a 5 footer or some where close when done,,,Hybrids will be same way,,,, 24 x 2 = 48 inches when done flowering ,,as a grower it will help you set the boundary's or keep them out of your lights nothing worse then coming up towards the end and not having the room for the plants to stay in the room or space,,,this works better then saying one is so many days old as plants develope at different rates so two of the same plants same strain can have a marked difference in how they grow but the size will remain the same,,,,,,,such as if you know your plants need to be 30 inches before you start to flower you know when they are all ready,,and as for training you can top a few of the tall ones to allow for the others to catch up so your grow stays uniform and same size