I also like using a percentage of the block size, but be flexible to also include the finished size you want it to be. Personally I would add a 2.5" strip around the pieced center using the background fabric. That will center your blocks and float it a little bit. The border would finish at 2". Next I would do a border in one of the reds if you have it, perhaps 4.5" to finish at 4".. Lastly I would do a bit narrower in the black to frame it all, or perhaps a dark green if you have it in the fabrics used. Cut this at 3.5" to finish at 3". I would bind with the same dark fabric, or with the red if you want it a little brighter. You can also switch positions of the 2nd and 3rd colors if you like it better that way. I would still bind with the dark green/black.