R25 sets the width of the sweep. You'll find some issues that use 3M9, for a narrower sweep, ideally suited to faster rates, and some use 3M3, for a wider sweep. Keep i mind that MXR aimed for a one-size-fits-all approach with that pedal/design. One fixed sweep width, one fixed wet-dry mix, one fixed feedback setting. When people express a preference for this issue or that, they selecting the P90 issue where the fixed feedback and width settings more closely match what they usually aim for.
By "wider", I mean that each cycle sweeps higher up before coming back down again. Typically, on any two-knob modulation effect, whether flanger, chorus, or phaser, the faster you set it, the narrower you set the "depth" (more accurately, sweep width) control. The usable range would seem to be between around 2M5-4M5, so a 2M2 fixed resistor with a 2M log pot would provide a usable range of sweep widths. Alternatively, a 4M7 default value, with two other fixed resistors strapped on in parallel, via a 3-position toggle, would provide 3 different sweep widths, tailored to different LFO rates. For a higher-feedback slow sweep, you'd probably want somewhere in the 3M-3M3 range. Of course, with a 2-stager like you show, there IS no feedback, but still one would generally like a less boing-ey sound from faster sweeps so a narrower sweep width helps.
