flow rate for peristaltic pump

There is only one way to get a decent answer and that's measure it.
So much depends on the viscosity of what you are pumping (ah! but you stated water), the back pressure you are pumping into, the length and bore of the inlet pipe*.
If you want a theoretical answer it will be approximately the swept volume of the squeeze tube, which will be half the circumference of the circle described by the tip of the rotor, times the cross sectional area of the tube bore. Mulpiply this by the number of revs per minute (or hour) to give the flow rate.
(Taking the tip of the rotor rather the centre line of the tube will pobably allow for some of the inefficiencies elsewhere).

*People think this isn't important, but bear in mind you are only sucking at atpospheric whereas you can pump at several bar. Even a sticky 'foot valve' or clogged strainer on the inlet will seriously reduce your flow rate.

Measurement is so simple quick and accurate and gives by far the most authoritative answer.
Del