What is a “reasonable” out of square angle?

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I'd recommend buying a slop top for your guide rail.  It removes the play where the track meets the locating pin.

People don't mention this frequently enough but you need to re-square everytime you adjust the height.  Also, check your fence and make sure it's straight.  I've had 3 MFTS and one had a bowed fence.  If you use a small square to square it, it'll look square but then when you cut a longer panel it won't be.

To be honest though the Festool system isn't ideal.  There's a user here that was making/selling a nice kit but it escapes me at the moment.  There's also a very nice DIY solution using dovetails in the MTFC thread.  It's maybe 4-5 pages from the end.  You'll have to scroll through.

Thank you for the suggestions! I forgot to mention that the slop-stop was already installed. Also, I did use the Starrett to check that the fence is straight. I did hear about the need for re-squaring upon height adjustment but, so far, after careful checking, I have not experienced it. My 4-cut test piece is 17"x17", which I understand, should be sufficient. My main concern is mitered joints for edges on the order of 18".