This thing was made with Tinkercad. Edit it online https://tinkercad.com/things/gMlCjUsQp39
I have been able to mostly compensate for the Y-Axis droop in a Printrbot Simple (Beta) by renaming the slic3r.pl that I use to slic3r.orig.pl and placing a simple Perl shim script as slic3r.pl which subtracts a portion of the Z coordinate from the Y coordinate for all vertices.
The model here is what I use to test the calibration. It has a nice, flat base to put a T-square on and thin walls to avoid using too much filament.
[UPDATE: see instructions for a better alternative]
[UPDATE: A firmware update is available as a superior alternative to the Perl shim script. See here: http://www.printrbottalk.com/forum/viewtopic.php?f=106&t=4425&p=37714#p37714 for details]
Level your print bed and print this part. It takes about half an hour.
It should come out as 70mm high (calibrate your Z-axis steps/mm if it doesn't). Check to see if the base is perpendicular to the wall corresponding to the short side of the triangle. However far off the top of the wall is, divide that by 70mm and adjust the value of $TAN, at the top of the shim script by the result (add if the wall leans towards positive Y... the interior of the part. subtract if the wall leans towards negative Y).
I would bet the value of $TAN that you use should remain between 0.0 and 0.1 unless you have a serious issue with your printer. The value that works well for me is near 0.03. Check the visualization of the sliced results to be sure all is well (it will appear to lean on the display, but the part should come out well).