Use a flash guide number to find the subject distance for a given aperture, or the aperture for a given distance, scaled for your ISO.
Guide number rates a flash: GN = aperture times distance at ISO 100. So distance = GN / f-number, and a higher ISO raises the effective GN by the square root of the ISO ratio. Use it to find the aperture or distance for a correct flash exposure.
It rates a flash output as GN = aperture times distance, quoted at ISO 100 in metres or feet. A bigger guide number means a more powerful flash that reaches further or allows a smaller aperture.
To find the aperture, divide the guide number by the distance. To find the reach, divide the guide number by the aperture. For 20 (m, ISO 100) at f/4, the subject distance is 20 / 4 = 5 metres.
Light gathering scales with the square root of ISO, so the effective guide number is GN times the square root of (ISO / 100). Going from ISO 100 to ISO 400 doubles the effective guide number.