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don't do that, and instead use UOA, turn on target queing, and use your mouse wheel.
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If you turn that on, make damn sure you also set a macro to erase the target queue!
I agree that you shouldn't set a "CAST xxx, WaitForTarget, TargetXXX" type macro either. For some reason, when you get interrupted, the macro can get hung up. Also, if you keep holding the button down or pushing it too quickly, you will wind up overcasting and losing spells.
Best bet is to put your target self and last target macros on your mouse. Nail whatever spell macro you want while spamming the desired mouse button (many people use mouse wheel up/down).