You should determine how much water is flowing in the zone and in each PEX run, the head loss in the PEX, and the temperature of the outgoing and return fluid.
There will be a large variation in heating in the zone if the temperature of the return water is very much less than the outgoing water. 300 ft is the maximum recommended length for 1/2" PEX in a slab heating system.
If the runs of PEX are approximately equal, the pressure loss in the PEX will be so much greater than anything else in the system that the flows will be pretty well balanced as installed.
How many BTUs per hour to be delivered?
What temperature difference between outgoing and return fluid?
Then calculate the required GPM in the zone and in each loop.
From the total GPM in the zone, calculate size of manifold. I would keep the velocity in the manifold not more than about 5 feet per second. The velocity head in both manifolds shouldn't exceed 10% of the head loss in the PEX to keep it from affecting uniformity of distribution.
From the flow in the PEX runs calculate the head loss in the PEX. The head loss in each run will be the same; they don't add.
Then add the head loss through the boiler and manifolds to determine the required head for the pump. Add 25 percent if you really want to try to balance each loop, which shouldn't be necessary if the loops are all about the same length and the loss in the PEX is more than about 60% of the total head loss in the loop.