Start by calculating the heat load of the room(s), then divide the total heat load by the floor area. The floor surface will deliver about 2BTU per degree F above the average room temp. The water temp required depends on the floor coverings and the heat-transfer plate types. Most radiant floor product manufacturers will give nomographs of average water temp in the tubing vs. BTU/ft^2 at different flooring-R. (From the nomograph it looks like with R0.5 thin wood or bamboo finish floor the WarmBoard product delivers 10BTU per square foot with 80F average water temps, but with an R2.5 rug flooring it would take 100F average water temps to deliver the same heat rate.)
Net-Zero homes typically have such low heat loads that the expense of hydronic floor heating is silly, with very little in the way of comfort as well. Using mini-split heat pumps for heating and cooling is often the most cost-effective approach, and is within the annualized output of rooftop photovoltaic arrays if you design & build air-tight with sufficiently low U-factors.