A suggestion for how far to stretch a tensioner:
The tensioner has two purposes, to take up the sag that develops as rain, dew or insensible perspiration causes the fly, tent, tarp, or canopy to stretch and sag; and to absorb wind shock to reduce stress on the fly and its components including stakes. If you pull a tensioner as tightly as possible, it has no reserve left to absorb wind shock. If the tensioner must be stretched to its full extension to get the tarp/tent set up properly, you need a stiffer tensioner.
Actually, tensioners have a third, specialized use: to keep hammock flies properly tensioned when your weight on the hammock would otherwise cause the fly to sag.