I'm making some waterfall joints with the TS75, and the 45 degree stop is out by between 1 and 2 degrees. I emailed customer service, and they just told me to send it in. I bought the tool used and believe it is out of warranty. Is there a way to recalibrate the bevel gauge without tearing the tool apart?

I do a large number of waterfalls on kitchen countertops (see 'TS60K' thread). Rule #1 = don't ever trust the numbers on the gauge. Make  a bunch of scrap cuts on offcuts first and see how your own saw is set up. I just bought a brand-new TS60, and out of the box, my perfect 45-degree bevel cuts are made with the scale set at 43.5 degrees. My TS55 makes perfect cuts set at 46 degrees.

I do this with a fair amount of frequency and have never had an issue with the angle being off. The main thing I had to learn was the sawing technique itself. You have to be very aware of the saw as it runs off of the end of the cut. It naturally tends to tip or maybe bow the end of the rail slightly, resulting in a miter that is not exactly straight. This may also be a side-effect of left-handed cutting and body position. It can be overcome.

@TomInNC
You can adjust the 45degree bevel stop.
If you look at the flat side of the saw, the one with the window to loosen the arbor bolt, there are two very small Allan key screws (2mm) next to the thumb wheels.

You can adjust those screws to set the 45degree stop.
There are also two torx screws on inside the saw, one behind the handle and one next to the front bevel gauge.  These screws set the 90degree stop.
You can adjust the green angle marker with a 2mm allan key in the small hole on the front of the saw.

Please be aware of what others have said, make plenty of text cuts when trying to set the stops.
Very slight changes in how to clamp the rail and hold the saw can throw off bevel cuts.

Good luck,
Bob

Edit, added some (very bad) pictures.

Disregard my previous post. Apparently I was writing that as Bob was putting together his detailed response. Thanks for that.

Is there a reason why Festool wouldn't include this calibration information in the manual? I swear every other tool I have has some line in the manual that states "it is adjusted correctly at the factory, but if it's now out of square, here's how to adjust the scale."

Some times I think Festool manuals are written by a competitor.

It's like getting driving directions from someone who knows where you need to go, then they tell you kind of like you have done it before, but need a little reminder. You get some of the directions and they assume that you know the rest.

My dedicated 45 degree angle picture framers’ miter saw is more accurate than any adjustable saw can aspire to.

That is partly because the angle is not adjustable and is difficult to knock out of adjustment. 

But the bigger part is that the adjacent cuts are made on the opposite side of the blade.

Assuming the 90 degree angle fence is truly 90 degrees, (it is), the left and right sides of the miter will always add up to 90 degrees, even if one side is 44 degrees, the opposite side is going to be 46 degrees and always adding up to a perfect 90 degree angle. 

I will step out on the limb and assume that most of the cuts are going to be to sheet goods of some sort. 

You could use cuts on the opposite side of the blades, but that would mean keeping track of all the pieces.  A headache I would not like to deal with.

But a router bit is factory fixed.  And like the dedicated miter saw, it cannot easily be knocked out of adjustment.

In that case, Whiteside (and others) make precise 45 degree angle router bits.  These are guided bits (with a bearing) but mounted on a table with a fence will be a better choice.  They make one (#2306) for cutting 3/4” thick materials and one for thicker materials (#2310).

It is probably best to first cut the miters with the saw and then just dress the edges with the router bit. 

The bit for 3/4” is about $50.00 and the one for thicker materials (#2310) is about $100.00.  The #2310 has a cutting length of 1-1/2”.  I am no good at square roots, but that should handle just over 1” thickness.

I should have mentioned it before, but I will primarily be using the TS75 on slabs, not on sheet goods.

For those of you that have used track saws for waterfall joints, what's your process for running through your test cuts? Do you do a 4-cut test? If so, how do you set the angle the first time when you are making a test cut?

I am new to the track saw world, but on the table saw or miter saw I would typically align the blade with a combo square blade to get the first cut, then adjust as needed. I tried doing this with the TS75, but when I used the FastFix lever to lock the blade at a plunge around the 45 degree bevel, I was having issues referencing the blade against the square because of the limited plunge depth. Is there a way to lock the TS75 blade at full plunge depth? Just trying to limit the number of adjustments during the setup.

I can get an iGaging angle cube on the back of the blade with the saw in the non-plunged position.  It's difficult to read the scale but it can be done.
regards
Bob