Again, this is adding variables to the test.

The problem with that adding weight is now you've changed the spine and FOC of the arrow depending on where you place the weight.

If you put the weight in the front you increase FOC and make the dynamic spine weaker. If you place the weight in the rear you lower the FOC and make the dynamic spine stiffer.

You'd have to ensure that you've kept the same FOC and dynamic spine or the test is invalid. For the dynamic spine you'd have to use calculations or a program to determine the spine.

A spine tester only measures the static spine of an arrow and would only be good to determine if your arrows all have a spine within acceptable ranges.