Got my r540s and a new cog (18t) in the mail
cleaned/lubed my chain, wiped down the frame, derotafixed the 15t cog on there, threw it on the freewheel side with a bb lockring, rotafixed the new 18t cog on the fixed side, threw the formula lockring on there - tried to see if the chain length would still work (key work try because my bb/crankset is off the bike, waiting for a new bb) = seriously 1 mm of space needed to get the chain off on the 18t side.

considering whether to go half link or a full link up, on the 15t side its already 50-60% out in the dropouts..