I have looked a bit into exposed pad sizes for the official library. To me it seems most footprints suggested by manufacturers suggest to use the nominal size of the “slug” for the exposed pad.
This seems odd to me as it does not take into account any tolerances (neither the tolerances of the part it self nor the tolerances of the manufacturing process.)
If i use the IPC calculations with 0 (or even with slightly negative) fillet i get a result that would be nearer to the maximum size of the “slug”.
The only thing i can find in IPC standards is a mention that the exposed pad should not be too large. And with too large they mean there should be at least 0.2mm clearance between the outer pads and the exposed pad.
Just to be clear: this is talking about the size of the exposed copper. (So the size of copper if a non soldermask defined pad is used and the size of the area free of mask if a soldermask defined pad is used.)
It is clear to me that the paste layer needs special handling.
With “slug” i mean the large metal pad on the bottom of such parts. (I chose this word as it differentiates from the word pad that is used to describe the PCB side)