I don't know what other equations or online calculations you used but I have reviewed all of the elements of the your above calculation using my Machinery's Handbook and don't find any errors in your solution.

At the same time, since the only loading you are applying is the weight of the tubing itself and that distributed weight is only 3.05 lb/ft (per Ryerson Product Listing) I am not really surprised by the resulting small deflection. I also calculated the bending Smax at the center of the beam using my Machinery's Handbook and that stress is only 788.23 psi; which would correlate with a low deflection for the beam.

I have also verified the above formulas in my copy of Mark's "Standard Handbook for Mechanical Engineers" so I believe you can be confident of your above solution

I also looked at an online calculator for aconcentratedload and saw an excessive deflection result. In reviewing the formulas shown for their solution I found that they were using "3" rather than the correct "48" as a divisor in their formula (an error factor of 16 times excessive deflection result). My conclusion being: Be wary of online calculators.