As Art' says, when tillering the bow, try to support it where the pressure of your hand will actually be and draw the string back where your fingers will be, this can make it tilt allarmingly at brace and low draw but gives a more accurate picture at full draw. Sometimes you can't exactly simulate a real draw, but it may help you get a feel for whats happening.
On my tiller rig the bow is free to pivot, not clamped solid, this also helps simulate the real life situation. The hook that pulls the string back on my rig is a two pronged one as wide as two fingers to help simulate real life, I have even put a glove over it (just kidding
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Basically if you shoot 3 under, then tiller three under!
Del