Almost all the time the buds on a cola will be the thickest heaviest largest buds on your plants so I cannot tell you how to match them on the lower portions of your plants. You can still get large buds below the cola but matching cola buds or topping them is not the norm. Those buds closest to the cola may very well come very close to being the same size if your conditions and care is right but often times even in them you will see some decrease in size.
As to when to flower you can of course flower at any time because the hormone that is triggered by the longer period of darkness, 12 hours or more, is in every marijuana plant at all times.
Picking some precise moment will not assure that you will have large buds on the lower portions of your plants that will match in size or exceed in size those on your colas.
Plants will double to triple in size, even more in the case of some pure sativas, while in flower so look at your height limitations, if there are any, and your lights ability to provide adequate light to the middle and lower portions of your plants and then decide when it is best for you to change your light cycle and induce flowering.
If you have the space/height and lighting, and of course properly care for your plants, vegging them longer will give you larger plants with more branches and more buds which will equate to a heavier yield but do not expect many buds below your colas to match or exceed them in size.
Now it is not impossible because it can be achieved with totally optimal lighting and the best of care and feeding but most growers, even top growers, do not provide such optimal conditions where as you go farther down the plants the bud size will not decrease.