A year ago I also tried to make some tracklog analysis in excel.
What I found:
1. It toke a lot of time until my boat reached the speed she had before the tack. t = 60 sec.
2. the acceleration is not continuous, poor helming...
3. My speed went down dramatically (21km/h --> 4km/h)ds = 17km/h = 4.7 m/s
4. This tack cost me (measure the blue area, just approximately ) ds x t / 2 = 4.7 m/s * 60 s / 2 = +- 140m.
5. So If I make an unwind leg with 5 tack instead only 3, it cost me nearly 300m. So I really need a good reason to do this.
Of course all of this depend on the wind/wave conditions, and just one tack is not representative. Based my practice the loosing should be much less. I hope...