Answered upthread.
If the inner 9-patches are 3" FINISHED, the S-in-a-S in the outside border, in conjunction with a correctly-sized inner border (to make it all flow smoothly), would be 4.25" FINISHED
3 x 1.414 = 4.24 , rounded up to 4.25" (and that rounding will cause a minimal amount of easing when attaching the borders)
The 1.414 is just another of those frequently used numbers (like the 7/8" and 1.25") that is helpful to know., especially when dealing with on-point calculations (either way).
Just make sure you always start with the FINISHED size of the block.
Last edited by MTS; 11-15-2012 at 05:42 AM.