A solid starting point is Distortion 5, Filter around 3–4 (bright — the Filter is reversed, so lower = brighter), and Volume at unity. Lower the Distortion for a crunch, raise it for a saturated lead.
Which way does the RAT Filter knob work?
Backwards compared to a normal tone control: turning the Filter UP rolls off treble (darker), turning it DOWN adds treble (brighter). Keep it low-to-mid for a cutting tone.
Recommended starting points are community-researched estimates — every pedal, amp and guitar is different. Dial from here, then GuitarToneAdapt helps you match a target tone on your own rig. Pedal comparisons · Amp settings · All tones