The airstripper gap question can't be answered in a vacuum
. Experimentation with your system will help answer that question, but the answer is, it depends on the configuration of the rest of the system.
I would start with the length of the pellet you want to shoot, so the head of the pellet starts being protected from the uncorking blast as soon as the skirt starts to clear the muzzle. A smaller gap offers more initial protection, but allow more air through the air stripper bore after the pellet. If you doubt that, think of a very small gap. The air wants to flow straight across the gap, with not enough of a "leak" to strip off much air.
The application and power level for caliber matter. Low power for target shooting and you might find the sweet spot for the gap at one caliber air stripper gap. For higher power, the sweet spot is likely to be two calibers, or more. How can that be, as higher power means more air? If the pellet is travelling twice as fast, it spends less time in the "blast zone" before travelling into the more air-still shadow after the air stripper lip. Or, looking at it this way,
at twice the velocity, a pellet spends the same amount of time crossing twice the gap as one at half the velocity, crossing half the gap.
The wider the air stripper gap, the more air it strips from behind the pellet: It is the instantaneous bulk air pressure ahead of the muzzle that drives the radial dispersion of that air into the lower pressure ambient. Actually, that "ball" of air is also expanding forwards, faster than the pellet ahead of it. The air stripper is intended to block the portion of that air that is trying to overtake from behind the pellet and upset it its "flight". At low muzzle pressure, more of the air from the muzzle is trying to follow the pellet, but there is less air in total to buffet the pellet. So, a bit of "swings and roundabouts".
If you look at shrouded PCPs that shoot at "hunting power", their air strippers are often 1/2 to 3/4" ahead of the barrel muzzle. Here, the goal is to reflect as much air as possible to the rear of the shroud for noise reduction, while still yielding field acceptable grouping ability. My observation is that such an air stripper can be spaced further from the muzzle without hurting groups at a given power, if the shroud ID is larger. Now, open air stripper function purely to minimize groups, so their spacing is optimized for that, with no consideration on the effect on sound - as they have very little to no effect on that.