The strand width calculation in the original post looks about the same as what I use now for kangaroo hide work. As for strand length, Paul certainly gives some sound advice.
For my own two cents, when I started making whips I first took all of the measurements right out of Dennis Rush's book, which I got through Tandy Leather Co. about 12 years ago. After I had cut a few more sets, I began using this rough method:
I would look at the core to be covered and then cut 1/4 of the strands to be as wide as the diameter of the core. So for four-strand work, cut each lace the same width as the core. If you want a steeper-looking seam in the plaiting, cut 1/4 of the strands to be the width of finished piece of braiding (estimate this width by wrapping two pieces of leather around the core). I found this to work pretty well for cutting by eye and it's still the method that I use to cut 4-plait bellies in bullwhips. Actually, I also use this method to cut the overlays for my cowhide bullwhips (see my earlier post with pics).
The only thing to remember is that your final width is always determined by how much you pare off of the strand when you pare your laces. When I cut, the strands aren't all perfectly the same width, but when I pare I pick one as the model and then I pare all of the other strands to be the same width. If one's skinnier, I either pare all the other strands down or cut a new, wider strand to replace the skinnier one. The paring is what will make a cowhide whip come closer to approximating the look of a kangaroo hide whip.
Let me know if any of this helps,
-Adam,
www.winrichwhips.com