I almost always use a tarp in A Frame mode. For the ridgeline ties outs about 8' (cantenary cuts usually not as long in back). You will usually not use one cord to run under the ridgline of the tarp, but use the ridgeline tie outs.
On the sides and mid I cut my cord to around 2', and normally adjust them to around 18", shorter in bad weather, and sometimes a littl longer. If your sides are longer, you can place your stakes around obstacles in tight spots.
The tensioners are light and useful, making set-up even quicker. But the shortest cord length with be 1/2 of the cord length.