Maybe I read your post incorrectly, but you should do the corners first, and
then work your way towards the middle.  You can put one screw or so in the
middle if you want, but if you start in the middle and work your way out it
is very likely you will run out of  material.  Sunbrella stretches really
nice, but you have to have room to stretch.  If you start in the middle you
limit the materials ability to stretch along its entire length.

John
87 West SVX with a new tent