You don't want to have the back too tight that once the clips are released the backing fabric will retreat to it's original shape which will cause the top to "bubble". I would always run my hand along the top making sure the fabric wasn't too tight or too lose. You don't want it tight like a drum. Moving your hand lightly, you can see if the fabric is too loose if the fabric moves. You want it to look like it has just been pressed. Try undoing a clip if you expect it to be too tight and watch the fabric react. If it is correct, there should be little movement. This is a learning process just like cooking. Keep trying because eventually you will be able to tell the difference.
I LIKE your set up. HOPE your plywood edges and been sanded thoroughly so you won't get any splinters.