You need to get the instructions for setting up the TV cable from Bowtie Overdrives. It's best to set the TV cable with a pressure gauge plugged into your trans. It's available on their website, but it takes some hunting to find it. Without the gauge, you're just guessing.

You're correct in that there needs to be some tension in the cable. The normal way to set it is to set the cable a little bit tight. Then back it off until the pressure stops dropping. Then you tighten it up until you get about 5 psi increase.

You can guess if you want, but the only way to be accurate is with a gauge.