How much deflection to expect...

[Jonathan]

You'll be fine with those rails to start with. I'm not saying the rails aren't important, but it's the whole picture not just the rails that affects the overall stiffness and the rest of your design looks pretty strong (as it's a fixed gantry) so you'll get away with it. For example, I cut plenty of aluminium on my machine before changing the Z-axis rails to profile rails and didn't notice much difference from the profile rails as the weakest point lies elsewhere.

[routercnc]

You can look at it another way which is if the machine stiffness is 1000N/mm, and you are cutting aluminium (guide figure 50N load), then you would see 0.05mm deflection at the tool (50/1000). This assumes a rigid tool (as your data was from the collet), and ignores resonances which would add slightly to the deflection in a difficult to calculate way.

If you can't be bothered with the maths - I just checked our kitchen scales (the flat digital type) and they go up to 5kg (50N). So if you have something similar handy use them to push against the collet until they read 5kg. If the DTI reads less than 0.05mm you are in business (>1000N/mm).