I could have written that better, I meant that I use polyester cord and that the same idea would be valid for webbing ... that is pull it a bit tight by hand, knowing that it will relax a bit under use. Obviously if you are using something on the webbing to tighten it up ... that changes the game a little.
One thing that I don't like about most production seats is that the edge cross rails are too close together ... basically the seat pad is too narrow and you end up sitting on those rails. Maybe go to a little extra work and think about if this seat is comfortable for you and if it isn't, then consider just making a new one, it isn't a lot of work to have something that is a lot more comfy.
Brian