The rocket is 27" long and I was going to set the forward guide at 6'" in front of the aft guide. I just wanted some input on if that is a good spot. at the CP it would be under 5" and I was told not to go below 5".
Bottom / aft rail button as low as possible to maximize the amount of time your rocket is on the rail. For practical purposes, that usually means on top of the aft centering ring (if you are anchoring into the ring), or right above it.
Top / forward rail button would ideally go at the point of CP (center of pressure), especially if your kit has large fins. However, if CP is too far from the tail-end of the rocket (e.g.: your airframe is long), you may want to position the FWD button closer to the aft of the rocket. Again, the goal is to maximize the amount of time your rail buttons are engaged and are keeping the rocket straight. For practical purposes, anywhere from 5 to 8 inches ahead of the aft button should be enough.
I have always heard the forward rail button should be at or near the CG...a rule I have not always followed but have not been snake-bitten by either.
Since CG should be 1-2 diameters ahead of CP, focusing on CG may push the FWD rail button position a bit too far forward along the airframe, minimizing it's usefulness.
The purpose of the rail buttons is to stabilize the rocket from two forces: uneven thrust of the engine (would rotate the airframe around CG) and the wind gusts on the pad (would rotate around CP).
Once the rocket goes fast enough, the fins start compensating for thrust peculiarities. So buttons' primary purpose is to act in place of the fins, until the rocket gains enough speed, thus the focus on CP over CG.
IMHO,
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