There is to much variation from car to car - YOU HAVE TO DYNO FOR AIR FUEL. If you can't easily dyno don't play with it. The plug you want to use is the NGK #FR5-1. This plug will dyno exactly like the stock plug when off the bottle. The stock plug is an accident waiting to happen.
At approximately 125-150 plus rwhp the propane pressure drop and the delivery system will limit the fuel enrichment and you will start to go lean. Gen I models are even more limited in the ability to supply fuel. BTR offers a gas enrichment kit for further hp.
Once you get more fuel into the engine at approx 50 additional hp you will find that the N20 delivery system starts to limit your hp and the car will start to run rich as you increase both
jet sizes. At that point changing to 6AN lines and a high flow N20 solenoid will allow you to continue with N20 up to about 60-63 jet size. At that point the sprayers are limited out. There are multiple scenarios and my comments only refer to single stage sprayer systems.
Your goal should be to get the max hp and still stay at 11-11.5 air fuel. This gives you some safety factor, plus the dynamics of acceleration and ram air will drive you leaner when on the track. I agree with MES, the VEC1 works fine without the bottle, however, it has limited use on the bottle - your a/f should be dictated soley by the jetting.