Power ring
If possible, dual power rings scheme as you described is preferred for noise isolation between IO and core. IR drop is one reason to decide the ring width. Another important reason is electro-migration issue. For example, if Metal3 can guarantee 1mA/um reliability, and your core consumes 20mA DC current, then 20um is the min requirement. Considering transient response, at least 3 times of 20um should be used. If the power ring is composed of multiple metal layers. The total width should be 60um.