There many sites online that publish generic spacing charts. I start with these, and then find a good average to the numbers that is slanted based on the action of the blank I'm using. The bottom line, though, is that the guide locations are preliminary. You REALLY need to do a static test and make sure that the load is distributed evenly along your blank. If you don't, you could set the stage for a fracture. At the very least, you will be robbing from your rod's performance. The only exception to this would be if you get a spacing chart for your specific blank from the manufacturer. But I would probably still put a static load on it to be sure. It doesn't take that long.