Lots of good discussion surrounding this question. Assuming you are planning to replace the front tires with 6-12 R1 ags, you will soon know whether or not you have the correct front/rear ratio when you attempt to turn a sharp corner. When you have sufficient "lead" by the front tires, they will pull the tractor through the sharp corners. If you don't have enough "lead" with the fronts, the back wheels will tend to push the tractor through corners, causing the front wheels to bulldoze their way through, making for a wider, not-so-clean turn. Having too much lead by the front tires will cause them to chew their way through corners in an over aggressive manner.
Please provide feedback to this forum regarding your real world experience once you have both replacement sets of tires in daily use. This tire replacement issue could become more of a concern, especially to owners of tractors with 18" rear R3 turf tires, as they have all but disappeared from the marketplace. Knowing that 16" replacement R3s and R1s are an option should be welcome news to anyone trying to locate set of 18s.