There's no need to "guess"... We have a manualyeah if it's a Marshall amp and I'm using a huge oversize recto 4x12 IR with Recto large speaker curve I'll put it at .5, to make the 16 ohm default Marshall see what the cab would be, 8 ohms. You can go father than .9 sure, it will just scoop and scoop until there's nothing left.
I think speaker impedance is a scale control, so if it's a mesa 8 ohm amp, 1.0 is 8 ohms and 0.5 is 4 ohms, idk. i think each amp loads reset with its 'normal' impedance, so mesa stuff would be centred at '8 ohms', Marshall stuff would load centred at '16 ohms'. Just to my ears and fingers when you reset Recto 2 it doesn't sound/feel like 8 ohms to me, .9 feels like 8 ohms, speaker impedance knob is pretty sensitive. if i want any more speaker impedance scoop i usually bring speaker LF resonance freq down towards ~80 Hz or whatever spot feels comfy for a given master volume setting, low enough to give it balls and clear up any blurry boomy undesirable mud