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Over at least a couple of decades, I've been making challahs. To get to your desired answer, I'd think you want to control for a lot more variables. For example, I made three equally weighted challahs last weekend, as measured by my scale. I divided each lump of dough equally into thirds. I then allowed them to rise for different amounts of time and at different temperatures. With just this data you should already be guessing that the loaves, while weighing the same as unbaked dough, did not weigh the same coming out of the oven and were of different lengths going in and coming out. They were all 3-stranded braids. But sometimes I make 4, 5 , or 6 stranded loaves.
Got my drift?
I suggest that you will not discover your answer with only the data you're asking for. I think you need to have all the loaves weighed (in grams, were I in charge) the same, divided into thirds (never into more than 3 strands) and then always rolled out to the same length before the final braiding is done. Then they always have to be allowed to rise at precisely the same temperature for precisely the same duration and maybe with fairly equal amounts of humidity and air-movement during the rising. Then the oven temperature would have to be identical. There are probably other variables you'd have to control for as well.
Doing science well's a bitch.
beads and then using twist ties or rubber bands to adjust the width and length of the "threads" and make some estimates. You could come pretty darn close and have more samples for your statistics. Everyone braids a little bit differently. Some braid loose and other compact, some long and some pull the strands threads while braiding. I predict you will come out with some general statements like the length will be about one third or one fourth the length of the thread as a rough estimate. Even then the entire braid can be squashed end to end to fit into a bread pan or stretched to change the width & length of the braid to get two or three to a sheet. Just something to think about.
I take it you are looking for open flat straight braids. No sickles, circular wreaths or panned loaves that pop up in the holiday season? Have you seen those super braids that are wrapped up like a snail and placed into a round cake pan? That's a very sophisticated effect and simply done. Don't know how long the braid is off hand but it takes some of the guess-work out (cause it doesn't really matter) and looks like a lot of work! Sometimes the unpredictability of braiding has led to some great solutions!
Sorry to complicate matters. It's in my nature to be creative. Good luck collecting statistics. :)
@ibor:You will be much farther ahead just to bake your own bread and gain experience doing it. There are going to be far too many variables such as type of flour, ingredients, room temperature, rising time, kneading activity, atmospheric pressure, humidity, oven temperature, braiding methods & sizes, etc.Experience will be a better teacher.Terry ThomasAtlanta