Both formulae are widely used and they are fairly accurate. The calculation for the 1/4m time is not quite as accurate as the terminal speed calculation because of gearchange speed of the particular driver. The terminal speed formula is not as much affected and the result is closer. Even so the difference is not dramatical.
I personally use the terminal speed calculation and confirm it with the ET formula.
The assumption of 18% drive train loss is for front wheel drive a bit high but for the 4x4 certainly a bit too low.