i dissagree with the air build up idea. less water = less air. as long as the overflow box keeps the syphon. it could be pushing one gph and the syphon would keep or even 0gph. if the design is right you wont have a prob. add a little water/air pump on the u-tube and then you can forget about a syphon break ever happening in the first place.
feel free to dissagree, but i'm telling you i know it happens... yes adding a small suction or dosing pump to the u-bend can help reduce and or eliminate the possibility of it happening, but i'm telling you that below a certain flow rate, air will accumulate in the top of the tube, and over time, will cause the syphon to fail...
in theory, your right, it shouldnt matter, but this is the real world...i've seen it happen, iave had it happen.
as for calculating headloss, you can generally just look at the specs on the pump and it will tell you that at 0 feet it can move ie. 500GPH and at 10 feet it does 250GPH, so, if the top of your tank is 5 feet, then you can pretty safely bet that it will do close to 375 give or take(it'll be a lil less but close enough)
and actually, if you really wanna get into it, there are tables that give the specific losses for each bend, size of tubing, and soforth, but i havent found a real need to get that accurate yet...