How to control velocity in a conical pendulum

[May]

Homework Statement

I'm doing an EPI on horizontal circular motion and for one test the independent variable is mass and I need to control the velocity by using the 2πr/T formula. So I know how to use the forumula to find an unknown but how do I use it for two unknowns (r and T). Is there a forumula with M in it that would help?. Because the r will be different for the different masses and so will the T. I'm really confusing myself now, but I guess my question is how do I control velocity in the experiment.

IV: mass, length of string
CV: velocity ...
DV: angle

P.S I'll be spinning it by hand and don't have access to a velocity measuring device (unless its an app or something)

Am I missing the point completely?

Thanks for any help you can give

 

[Simon Bridge]

You use $v=2\pi r/T$ to find the velocity, not control it.
You can find r using some trigonometry and you measure T with a stopwatch.