ISTM you're getting too picky. In most helicopters, the seats are in the ship when it's weighed, so you know the empty CG, & exactly where the seats are doesn't matter when they're empty. When you put pax in them, all you need is an approximation - are they leaning back, leaning forward, are they fat, or thin, are they full back in the seat or sitting on the edge with a lose belt? All you can do is get an approximate central location & go with that. In any case, the CG won't change appreciably, & there's nothing you can do about them moving anyway.