Best practice for UFH is to NEVER EVER use the up and down route, you'll end up with hot spots and a heat gradient across the room. The ONLY time we have to do that is when retrofitting aluminium plates under an existing floor, and then we aim to run the flow round the outside walls first with the return being at the 'inner end' of the house, even so, we'll then aim to more, shorter loops in.
Spirals give an even heat distribution as you get the coldest (return) next to the hottest (flow) so you end up with a really even temperature distribution across the floor.
A good designer should also have given you the heat loss, flow requirements, approximate pipe lengths, and pipe spacing for each loop. If you have a large picture window / patio / bi-fold doors, they should also have closed up the pipe spacing there as that wil be one of the big heat loss areas.
The pipe we use has it's length marked on it, (counting down) so you always know how much is left before you start running the loop, and seeing as from above you know how much you need it's never a problem of running out. By then planning the installation order (sometimes working from both ends of the manifold) - typically we will have 10-12% 'wastage' off a 500m coil.
So length is the same ....
