That box that I described is for most Sikorsky models, which have pretty high power and thus lots of collective. Generally the rigging diagram is fully additive, so that the description is almost square - full up is 20 degrees, and add then about 18 to 20 for forward cyclic., but the last few degrees are generally cut off Max forward cyclic occurs about 90 degrees earlier in rotation, so the max angle would be on the left side.
Often, these are hard to achieve because of geometry restrictions, so that the extreme corners are cut off. One can argue that at max collective, the most forward cyclic you need (to execute the max pushover while at full aft CG) is much less than 18 degrees. Similarly with left and right cyclic. Real flight data from similar models and similation data is also is used, where the control points that were actually achieved or simulated are plotted to show that they reside inside the proposed rigging box. A glance at the maintenance manual rigging instructions will tell you the real story, and most of this is public data. I don't have that data, unfortunately.