Centerline wheel spacing numbers are the same regardless of tires. They usually specify min and max spacing called "tread." You can take the tire size and add the width number (14.9" for example) to the "tread" on the spec sheet and get the full outside to outside measurement. (Same result as what ericm979 describes above as half per side.) I have found "on paper" measurement to be very close. If you need closer "on paper" measurements then manually measure the real width of one rear tire at side points which you want to go by--- in other words whether you care about tread width, overall tire casing width, etc. In any case just add the tire width to the tread quoted in the tractor specs. The spec sheet for your 6060 on the Kubota site shows min as 52.2 and max of 56.3. With 14.9 tires you then have min width overall outside of 67.1" or max of 71.2." If your R4 tires are wider or different width than 14.9 just add the tire width to 52.2 for min and 56.3 for max. Note your spec table shows "overall width (min tread)" to be 67.3" which is 0.2" wider than just adding 14.9 to 52.2. I have no idea why. I doubt that your concern is in tenths of an inch anyway.