It's true that the shorter the ridgeline (i.e. the more sag you have in your hammock), the more tension the SRL will hold. Normally the hammock fabrick itself is carrying the tension. When you relieve that tension to create more sag, it has to be carried by something in order to transfer the load to the suspension and on to the trees.
It can be easily calculated using simple trigonometry, similar to the way the tension in the suspension is calculated. In fact, the SRL tension is going to be a function of the suspension's tension.
I noticed this last night as I was adjusting my SRL length. The shorter I made it, the tighter it got (and the higher pitch it would make when I twanged it with my finger). I found my most comfortable lay at about 80%, which was surprisingly MUCH tighter than my starting point of 83%.