There would be a minimum distance that you could have the broadhead from the rest and that would be for safety reasons. But, as far as I am concerned, there really is no maximum distance the broadhead should be from the rest (within reason). The length of your shaft should be determined by what you are trying to do with the arrow. Right now I am shooting an arrow with a shaft length of 30", the overall arrow with fieldtip is 31 1/4". When I put a Muzzy Phantom broadhead on it, it gets a little longer. The length helps balance out the spine stiffness so that the arrow shoots correctly out of my rig. I could have used an arrow with a weaker spine and it would have ended up being shorter but then I couldn't have achieved the weight and FOC that I was looking for.

Before making an arrow you should have an idea of the tip weight that you want to use, the fletching you want to use, the total arrow weight you are trying to achieve and the FOC you are trying to achieve. Then, depending on the draw weight you will be using you buy a shaft with a spine that is close and cut it to the proper length. Put the arrow together and it should shoot relatively well and then you can make minor adjustments (raise/lower the rest, increase/decrease the DW)to your rig to compensate for the variables in the arrow.

There is probably stuff I left out and hopefully others will chime in.