Yep, RT6 is a 0 offset. And with 285/60, you will trim a considerable amount. To avoid it, the 20mm will tuck it within a minimal trim pocket.

IIRC, my centerbore (18x8.5) is 108. The debate whether we are actually 100 or 108 is up for others while I just sit and back accept my happiness with what I have, heh. You should be fine as long as you don't stray too far from that range (under 100 probably not so good, higher, consider hubcentric rings).