Power routing cal is not a completely trail & error technique. yes , it is a bit of an iterative approach . its 70% calculative & 30% iterative.
Do an bottom up approach.
1. Calculate the amount of voltage required by your design(esp centre of chip).
2. now add the IR drop in power distribution network (rings, mesh , rail )
3. Based on a step 2 , you can calculate the voltage req at the starting point of power distribution network.
4. play with step 2 & 3 to achieve optimal results
since step 1,2 & 3 are straight forward but step 4 is iterative. that's why we call it a mixture of calculative and iterative approach.
based on your exp in design , you will be able to close the "iterative" loop sooner
Regards
Nav