Being slightly positive when vertical is normal. You can reduce it some by bringing the reg up a little closer to your head. Inches make a difference here.
The lever sounds a little low to me. First off, the reg MUST be pressurized when you set lever height. It will drop a good bit when you pressurize the reg so if you set it up unpressured, even if it's right, it won't be when you turn the air on. This is true for most regs. I pressurize the reg, adjust the lever to about even with the can lip, set the diaphragm on it and gently tap on it. I can gauge the spacing by feel, there should be a little play but not over 1/8 of an inch. An IP of 135 is fine but can be used as a tweaking adjustment for cracking pressure. Keep in mind, unlike a single hose reg, a DH is a balancing act of pressures, adjustments and positioning. When done correctly, you can get your PRAM to outperform most modern regs but it does require some tweaking and experimentation to get right, esp if you don't have any experience with DH regs.
Once you initiate airflow the venturi should be noticeable but not uncontrollable. If it's not , then start blocking one port partly, test and repeat until it's to your liking. The venturi is pretty effective so don't over do it.
One more possibility, is you BC. If you have any air in it and it's not snug on your body, it will float up from your back and make breathing difficult, again, inches here make a lot of difference. When horizontal, reach around your back and see if your fingers will go between your back the plate. If there is space, reduce your weight to remove the excess air and tighten up the harness.