This system is designed to furnish more heat than is truly needed. With this fact in mind, if you simply spread the tubing out as evenly as possible you can not go wrong. Concentrating the tubing slightly under bath rooms and areas of high heat loss adds to the overall comfort.
If you take 500 times the number of rolls of tubing designated for this floor and divide that by the length of the joist space, this will give you the number of single runs you can install. If the number of single runs exceeds the number of spaces, this number represents the number of doubles you can run. Be sure to allow extra for the run to the manifold.