The very best resource I found for understanding the requirements and design parameters of radiant heating was a book borrowed from my friends who installed their own radiant system a few years ago. It is a large paperback from Uponor Wirsbo. The version they lent me is from 1999 but it was all the basic information that was invaluable. I spent most of my time reading and absorbing this manual and used the tables and calculations to design the system.
The Uponor website has tons of installation and design manuals available for download, including an update of the manual for radiant design (CDAM). This is a great service and makes me glad I am using their products.
Even though I have read advice against using more than 150′ runs of 1/2″ pex, the manual says that 300′ is possible with the correct design for temperature, flow and heat loss. Actually 330′ is the specified maximum length for a loop but no more than 300′ is recommended in the manual. So the runs planned for the floor are all under 300′ but over 280′. They should be as equal as possible. Here is the planned layout.