attempting to be exact about your timing is like measuring with a micrometer and cutting with an axe.
It really is. There is just no way (sans GPS) to be that precise about ground speed.
I mean, you have IAS, which is going to be a little off of CAS.
Then you have TAS, which is going to generally be a few knots faster than CAS.
Then you have the wind, presumably a headwind. I've seen it vary by 20 knots or more over the course of flying final (not talking about wind shear).
And inaccuracies of how well you fly - holding speed, starting the timer, and even whether you're weaving back and forth across final or not.
Pretending that timing is anywhere near remotely accurate is a fantasy.
Fortunately, some of these errors tend to cancel themselves out in a lot of situations. For example, if 100 kias = 100 kcas, and the airplane is at 3000 feet on an 85 deg F day, then TAS is about 107. If there's a light headwind, then there you are back to 100 kts GS which is the same as indicated.