There's nothing wrong with this procedure, but you should keep one critical point in mind.
Before measuring the used CVT fluid, keep it along with the new fluid in the same room for a couple of hours to bring the new & used fluid temperature same. This will ensure your measurements are correct.
I am not sure about Toyota, but Honda's recommendation is to bring the engine temperature high enough that the radiator fan kicks in twice before you drain the CVT fluid. In such a case, it's even more important to bring the used fluid temperature down and match it with the temperature of new fluid.