First, do what Kaw tells you in your manual. Altitude will have a bit more effect on higher rpms than at idle.
Next, the petcock. Your vacuum actuated petcock has two lines... one for the gas input on your carb assembly and the other going to a vacuum port on the carbs to open the flow of gas when the engine is running.
You have prime, on and reserve? The on/reserve work the same way. There will be two fuel pickups inside the tank attached to the petcock. The ON pickup will be a tube that sticks up about an inch and the RESERVE will be from a tube that sticks up about a half inch. The effect will be that when the ON tube is not able to draw gas because the gas level is below the tube, the RESERVE pickup will. This is a safety or convenience feature so that you will be less likely to run out of gas. In this sense, it doesn't much matter if you operate the bike in the ON or RESERVE position except if you run out of gas while in RESERVE, you will soon be hoofing it.
The PRIME position is there so that if your bike's carbs are dry, you can gravity feed gas in since there will be no engine vacuum to draw gas in since the bike won't be running. There is some vacuum generated when the bike is turning over with the starter but PRIME is there to ensure you can feed gas in. The reason they have a vacuum actuated petcock is that folks used to have to remember to the the gas off when parking a bike. The reason is that the float seat/needle are primitive stoppers and some gas can dribble by and puddle under your bike if you were to forget. The vacuum actuated petcock allows riders to just park their bike and have the petcock automatically shut off. That is good but starting with dry bowls is made a bunch more difficult without the PRIME feature. ONLY use PRIME when your bowls are dry to "prime" them then switch to ON after getting it started.
