The old swing of the curve test ... NMRA has standards for such things .....but I think they were only considering 1/4" scale .......put a 17/64ths piece on the same curve ...the gap narrows considerably.....so test with the biggest thing you have then add a bit ....
As a collector of vintage O scale ... I run both 1/4" and 17/64ths ....all clearances are done with a Mini-Scale 17/64ths Pullman ...biggest thing to roll the rails . Here are photos to compare gap with 1/4" VS 17/64ths
Cheer Carey