To get the flow at a particular speed, you need to know the total head, which is the suction and filter pressure added up and multiplied by 2.31.
We don't know the suction pressure, so we will approximate it at about 1/3of the filter pressure, which may or may not be accurate. You would need a vacuum gauge to tell for sure.
For example, if the filter pressure is 15 psi, we will estimate the suction to be 5 psi for a total of 20 psi. 20 psi x 2.31 = 46 feet of head.
At 46 feet of head and 3110 rpm, you should get about 124 gpm.
You have to estimate where the curves are for the speeds between those shown.
You don't want the operating point to be at the top of the curve where it's flat or all the way to the right of the curve.
58 gpm at 3450 rpm puts you at about 93 feet of head (about 40 psi) which is too close to the flat part of the curve.
32 gpm at 2350 also puts you near the flat part of the curve, which indicates too much restriction on the suction or pressure side.
What is the filter pressure and gpm at a few different speeds including those in the graph above?