500 seems OK to me, if the cold idle speed is stable and the OP shows at least 10 psi.
In addition to the actual factory curb idle solenoid, which are pretty expensive new, you can bodge almost anything to work from another brand - all you need is to place the plunger (when extended) in-line with your existing idle stop screw, and slightly higher to raise the speed when the solenoid is hot.
You can use the really cheap (like $10.) 12 volt commercial surplus parts, but you want "push = hot" (not pull, unless you can make new linkage on the other side), and "continuous duty" since it will be hot whenever the key is on.
Here's a good pull type:
https://www.surpluscenter.com/item.asp?item=11-3179&catname= 
Now, the solenoid plunger is your new idle speed adjustment; if it's not threaded you can move the entire solenoid, etc.
Lower your original idle speed screw several turns, since it's now the "no air, anti-diesel" position with the ignition off.
