Eric
You can go way lean, (for more power) since your only running 500'. I run 150's at sea level, and 148.8's at 1200' of elevation. Then I like to adjust the needles rich, so when you slam the throttle wide open, there isn't a lean bog. I usually run my needles in the #4 position.
For gearing at 500' I've had good results with 20-39. As for the secondary spring, I like to run it as loose as possible without sucking the belt. This allow's for quick up shift's. This is where testing comes in, start out with the spring fairly tight, say at 90 then make a pass. Then start backing it off 10 degree's for each pass. When you get to the point where rpm's are ok at the start, but fall off a short distance off the line then slowly climb back up, (sucking the belt) you have gone to far. At that point you would want to tighten the sec back up untill it no longer suck's the belt.
Always tune the shift rpm's with primary clutch. When tuning rpms (primary), I like to keep throwing more mass in the primary weight's untill I reach the point where the motor wont take anymore, and falls out of the power band. Then back off the weight amount untill she's back in the sweet spot.
This requires a far amount of testing, but's it's the only way to know for sure that you have the most aggressive clutching for your sled/conditions.