Take a 3' level hold it against the sides on both the top and bottom limb pockets match the sight bubble with the level bubble and your set.
I have done it that way in the past but do not do it anymore. the level is really there as a reference point so that whatever angle you hold your bow at, it is consistant.
Personally on my Darton, it is a little more conforatable for me to hold the bow at a very slight angle (cant). An adjustable level would let me conpensate for that.