Volvo 780 5th wheel position for flatbed

I just got this truck a few weeks ago. I realize the ride height valve may be bad. Sometimes takes forever to fill or exhaust air. Dumping from 12psi to 0 took what felt like a long long while. Noticed the front airbags looked like they leaned forward a little. So I slid the 5th wheel forward and it seems like it straightend the airbags up some. I only moved it a couple notches, so it's a little forward of dead center of drive axles. Closer to forward axle now. The leveling valve looked like it wasn't at a 90 degree angle. Maybe 85-87. Where would you all put the 5th wheel? The trailer is a 53 foot flatbed. The 780 has the 13,200 lb steer axle rating. Thank you in advance.