Now that you have a good idea of what plait fits over what for you, you have the information to start planning your taper of your whip almost down to the millimeter.
You may already know all of this, and if so, please disregard

Start from the outside. Draw a line and mark where you want what plait count, where you want to twist, etc. That is the map of your overlay. Write your plait count above the measurement.
I don’t worry about drawing every detail for strand drops. Those will happen where they need to of you have accurate information in your personal experience of plaiting of what plait fits over what. You do have to remember how you build the inside, if you’re adding something or taking away something, will affect your plot map.
Next line is second belly. You simply draw another line to the point of where you know it must end in a twist or whatever style you need. This is adding the muscle layers under the skin of your “living braided snake.” You mark what plait fits under what plait that’s on your first line, or overlay.
First belly- same procedure. Inner layer of “muscle,” write your main marks of where it drops to a lower plait, where it goes into a twist or however you end your bellies.
Now the main and most important of all, the core, the “backbone” of your “braided live snake.”
Draw your line, plot your twist and number of strands needed (4 TT, 3TT, (TT being twisted taper) or whatever style you use for a core) for each measurement.
NOW you know what length you’re core needs to be. You can push the weight forward or pull it back by adjusting your core.
It is very simple to plan a whip this way, but it can’t be simply done unless you’ve built a few and know what type you’re doing (the internals, such as TT core, or single strands, how you end your bellies, etc.) and what plait fits over what in your own work. I may plait a 10 plait over a 4TT, you may use an 8, someone else may use a 12. That’s the difference in style of plait, tension, angle, and how you want the whip to feel.
You can plan any whip with that simple whip map.
I have a notebook for my whip maps. All I have in it is three or four lines for each whip. Nothing fancy. Simple main measurements.
You want to replicate to a longer or shorter model? Simply figure the core is what percent of the overlay. What main drop (4TT to 3TT, or your overlay and bellies 12 plait to 10 plait, etc) is what percent? Now extend or shorten by figuring your percentage of whatever desired length.
Your 6 ft has a core 50% its’ length, 36”? Then a 10’ will be the same, 50% the length, or 60”... just an example. Bellies and main tapering is figured the same.
Inch by inch.