Trimming the wonky corners of the Thelema deck?

Silmaril23

The fact that this deck has wonky/crooked corners is well established, so it did not come as a surprise to me when I unboxed it. I already knew I might have to trim it a little.

HOWEVER...I've never corner trimmed any cards before, and I now need to buy a corner rounder. What size should I use for this particular deck? It looks like 5mm is the standard, but I can also see smaller sizes (3mm? 2mm?)

I want to trim as little as possible...just want to standardize the corners, since they are atrociously jagged.