Fastcap magnetic micro square 45 degree or another square for tool setup

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It seems we are looking for perfection in measuring angles, but is that table saw blade dead flat? If you turn the blade by hand does the angle vary? Does the fence change its angle every time you move it and lock it in a new position? My Makita was really rhat bad on both issues.

Is the accuracy being sought more than necessary for everyday woodworking? Are we becoming obsessed with something we can never achieve?
For hand-made work the slight imperfections are what separates us from the mass manufactured stuff we see in furniture shops these days.

For completeness, I have a digital angle finder, as mentioned in other comments, and a several squares and the 45° gauge supplied with the table saw. I use whatever tool I think is appropriate for the task in hand.

Since moving away from that crappy Makita table saw and moving on to an Axminster Craft saw, I have had no discernable errors on the machine. All the errors were mine. I'm not heading towards obsessions at my age.
 
perhaps this ? or this
Alternatively you could buy a 45 degree square or digital gauge and send to an UKAS accredited facility to have it calibrated and factor in the error. They may or not be able to do it.
The Standard for Engineers squares BS 939 does not include 45 degree squares. I haven't looked but, there may not be a standard for 45 degree squares, so that makes manufacturing and calibration difficult for any facility. You could try searching for UKAS calibrated 45 degree square. For the accuracy you seem to require, you have to find a standard to aim for, then you know what to search/ask for.
 
If you want a small square and a 45deg reference, you need an engineering square. These are available in a ranage of sizes from very tiny 30mm say to very large 1000mm, at a range of accuracies. I got my small ones from Kinex (in Czech or Slovakia?) when we were still in the EU. About £10 each. If you look on their website they (used to anyway) show all stock sizes and accuracies and explain the standards used.
I have and like the Jap sq/45 shown for marking out. The 45 deg matches my reference but the 90 deg was out and reqired some correction, which was probably quite unnecessary for woodwork!
 
To the OP, I would also simply recommend a metalworking machinists square for 90 degree blade setting. Workshop grade 2 is far better than a bevel box. Grade 1 better still for a bit more money. As these are made to defined standards, the brand isn't very important. Kinex have a good range. Cromwell Tools own brand "Kennedy" is perfectly good.
For a 45 degree reference, I'd use the hardened, ground head of my old moore and wright CSM series combination square. That's smallish, has two ground faces at 90 and 45 degrees and is again made to greater accuracy than pretty much any tool intended for woodworking.
Use a light behind the edge when bringing the blade up to the reference. That will show up any mismatch finer than a feeler gauge can measure.
I don't think it's difficult to achieve your aim. You just need to buy one or two decent quality tools made for machinists / metalworking and look after them.
Interestingly, I have a stabila digital spirit level that is specified as 0.05 degree accurate at 0 and 90 degrees only (not intermediate angles). This is 4x better than you quote for typical bevel boxes. That device is no use for blade setting - way to big - but you would probably find 0.05 degree accuracy acceptable for woodwork if they ever make a bevel box that good.
 
I appreciate the advice.
I think I should explain something:

the 0.2 degrees in itself is not the biggest issue with the level box digital gizmo:
it’s the inconsistency. Means sometimes you will get 44.8 sometimes 45 or 45.2. I am also dead sure the margin of error on the particular unit I have got is bigger and it’s inconsistent;

Now onto the squares, I think will go for 2 separate machinist squares at the end, one 90, other 45. It would have been nice to have one with both of these, but machinist squares makes sense. Will be good to use as a reference to re-check my combination squares too.
 
Measurement uncertainty is a factor when measuring anything.
 

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