Ray, I set up my new MFT/3 this past fall and have a few suggestions.
1. Don't assume that the factory installed stops for the guide rail supports will result in a cutting edge that is exactly 90 degrees to the front and back of the table and parallel to its CNC'd holes.
I'm sure my table left Germany well-calibrated, but by the time it reached California it was off by 1/8th of an inch over the length of my guide rail. I found the manual a bit cryptic on this point and I didn't see it discussed in the Festool setup video.
Fortunately, before making the first cut on my table I saw Andy Guile's ("The Woodgrafter") MFT/3 setup video. At 6:34 of part 1, he discusses how to check the accuracy of the factory stops and make adjustments, where needed.
2. As mentioned above, a good square is essential and I picked up a 16" x 10.5" Kinex DIN 875/1 machinist square from Taylor Toolworks for what I thought was a reasonable price. Here's a link:
https://taytools.com/collections/brands-kinex/products/kinex-2-piece-din-875-1-machinist-squares?variant=18087098253363
3. I found that the tab in the front guide rail support (where it connects with the channel on the bottom of the guide rail) had some play in it. Apparently, this is so common that an enterprising person developed an inexpensive product called "Slop-Stop" to deal with it. I bought one and it did the trick.
https://www.ebay.com/itm/Tune-your-Festool-MFT-rail-with-Slop-Stop/274200711773
4. Peter Parfitt and others have suggested purchasing dogs for your table and I would definitely agree. Following Peter's advice, I've been getting accurate 90 degree and 45 degree cuts with my TS55 just using dogs and the calibrated guide rail. You can view Peter's excellent "New Brit Workshop" videos here:
5. Dust extractor? I tried going with a cheaper shop vac, but quickly broke down and got a CT Midi. The Festool vacs are very good.
Good luck!