You want to design your alternator for the max power point RPM of the turbine. This is computable from the turbine type and the velocity of the water as it enters the turbine. For a pelton or turgo it's where the circumferential velocity at the radius where the jet hits the spoon is (trivially under) half the jet speed (because at optimum energy transfer occurs when the water does a 180-degree turn in the receding spoon, leaves "standing still", and drops away by gravity (or a small amount of residual velocity) before the next blade arrives.) For a crossflow it's encoded in the angles of the blades where the water encounters and leaves them. For reaction turbines it's also encoded in the geometry of the blades. Don't forget the factor of 2 pi when converting from radius and circumferential velocity to RPM. (You can estimate any residual velocity of the water from the efficiency of the turbine type because friction with the blades is very low.)
The initial velocity of the water is easily computed from the operating head (the actual head less the pressure drop from friction with the penstock). You can compute it by computing the velocity of a free fall from the (effective) head. (v = sqrt(2*g*distance), g = 32 ft/sec^2, g = 9.8 m/sec^2) Compute the effective head from the actual head by deducting the pressure drop from the flow in the penstock pipe using a table for the pipe size.
You'll adjust the water flow (by sizing and selecting/valving the number of jets in a pelton or turgo, the opening between the nozzle and distributor in a mitchell-banki crossflow, and so on for other types) to adjust the torque - within the constraint of keeping the penstock full to avoid wasting head. This means the pressure / effective head is within a narrow range (varying only due to friction as you adjust the flow).
So your particular turbine will be at max power point in a narrow range of RPM which can be calculated in advance from head, flow, penstock size, and turbine design.
(You'll also be able to compute your torque, and thus your output current at a particular voltage and generator efficiency, from the available water flow, effective head, turbine efficiency, and RPM.)