I confess that I don't know the formula to the 6th decimal place. I use 1.94 and it works pretty well.
I am curious about your interest in the "exact" figure. Is this purely academic interest or do you think you will get more accurate speed calculations if you use a six decimal place factor instead of a two?
I don't think using a six decimal factor would increase your accuracy as I think there are more errors intrinsically in your timing (do you time from when the first pixel or your target firsts hits the first pixel of the cross hair or when the middle of the first pixel crosses the middle pixel of the cross hair?
Have you ever tried to locate the very last pixel that is rendered on the stern of your target? Aint easy!
I would imagine that computing variations in your graphics card, CPU, and monitor would negate any "accuracy" in taking the 1.94 down to six decimal places.
I would also imagine that the mods incorporate the historical inaccuracies of the TDC and torpedo performance.
For warshot situations, when pressed for time, I have actually used 2.0 instead of 1.94 and still poked holes in the floaty things.
So from a practical standpoint, I think that 1.94 is way too much mathematical accuracy as it is, taking the factor to six decimal places I don't think will have any practical use.
Now if this is simply an academic interest, you could easily find this number out with a piece of paper, ruler, and some simple trig. It would be an interesting problem to noodle out