I can't answer specifically for a rimless tank but when I was getting ready to drill a bunch of my plastic trimmed tanks I searched a lot online for recommendations.
One site recommends to stay a minimum of 1" from the edge of the glass and any corners. Another site recommends that from the edge of the glass to the edge of the hole, there should be at least the diameter of the hole in distance.
You also need to take into account the bulkhead fitting you will be using to make sure that the flange and gasket is supported or in full contact with the glass.
Eheim Pimpette #270
Eheim Pro II 2028; Classics Series -2215, 2217, 2260, 2262; Pro 2229 Wet/Dry