I have always been interested in collecting data on my machine’s performance, including measures of deflection.
For some reason today I decided to put a dial test indicator on the top Y rail to see how much the mass of my heavy ATC spindle+X50 X axis+3rd rail deflected the Y axis rails.
I zeroed the indicator (2 micron increment) at the top of the Y rail, at the midpoint, when the X axis gantry was at the front homed position. I moved the gantry as close to the indicator as possible and measured 80 micron (0.08 mm) deflection - which was repeatable. I did not measure the X because if three 50 mm hardened tubes flex over 40 mm travel then that is a whole other issue 
It got me thinking that for some of the precision work I want to do in aluminum, I may set up a work holding area closer to the Y axis support blocks, vs the middle of the X/Y travel as it is now.
Those that have read my posts over the years know I enjoy chasing microns, but I also have no grand illusions of the realistic tolerances I can or should expect from this hobby CNC router.
Just thought I would share - maybe others (will?) have similar or differing results.