That's alot of space to fill even with a spacer, but it sounds about right. If you take a peice of wood 1 inch thick and put it on your rotor, then push the rim up to it as close as you can it should give you a good idea of how far out it will be with adapters. Here is a website that might help with the backspacing.
https://www.rsracing.com/tech-wheel.html