I have never seen a Packayak in person, but I would imagine the back band has either a ratchet or buckle to allow the back band to be snugged up a bit. As a new kayaker you may not have your core muscles accustomed to sitting in a kayak for an extended period of time, but you may be surprised how quickly you will build them up.
I do not think there is a right or wrong tension for the back band, it depends on your personal comfort and ability to properly rotate your torso to use the rotation to power your stroke. Over time you may find a taught back band could limit your “lay back” when rolling a kayak.
Many kayaks are adjustable in many ways, foot pegs, back bands, seat pans, can be adjusted, any one of these adjustments may create the need to adjust everything to get the fitting correct. If you did not get your boat from a kayak dealer, you might look for a video on fitting your kayak in a general sense.
Through the years I have purchased (and sold) many kayaks. I have a bad lower back from a long-ago injury and in many cases I needed to re-engineer (or even replace) the back band. I need the back band to give me some support when I am in a proper upright posture, especially if I am kayaking for the whole day. I ensure that the back band does not inhibit a layback roll.
So when I buy a kayak I take a good look at the back band to determine whether I can re-engineer (or even replace) the back band. I cannot hope that the back band works as is.
Most of the magic is in having the backband low enough. It should be about as low as you can make it and still give you some support, think more sacrum than middle of the back. That way it does not get in the way of rolling or re-entry.
At the very least, tighten the backband so that you are not leaning back against the combing.
Optimal (but may require you getting muscled used to doing this) would be such that the backband is in a position where it touches you as you are sitting upright.
It is not really there to support your back, but more to offset when you push against foot pegs so that your body locks into the boat and becomes one with the boat. Without the backband, if you pushed on your foot pegs you would just slide backward.