i use a backplate setup without a crotch strap. The shoulder straps would have to be quite snug and fitting, but not too tight. An estimate would be to be able to put a fist between the shoulder strap and your chest. When fitted correctly, the BCD will not feel like it's riding over your head even without the crotch strap.
The different design also means you can't expect to float vertically with ease on the surface, as compared to a jacket BCD. you will need to lean slightly backwards.
Also, if you fully inflate a 40lb wing on a backplate it will definitely start to push you forward. So lean back, and enjoy the sun. Of course, if your head is barely above the water and you're drinking water with every wave, then inflate more lah.
So in summary - proper adjustments, different posture, control inflation.