A problem with these systems is that they only work for certain kinds of shots, such as two rail shots or three rail shots in which only the contact point on the last rail is determined. Various systems must generally be learned to cover different situations arising based on the position of the cue ball and the intended direction of travel.
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Aditional problems occur when the author of the system developed it on a table that doesn't play like yours and with a stroke that isn't much like yours either.
To circumvent these difficulties, and to insure that you will never face a shot that's not covered by a system you haven't heard of yet, I recommend the following: