Great question. It wasn't a 'by the pitcher' breakdown that I used, but rather a look at the whole group. Here's the math I used to arrive at 100 runs for the rotation:

The 2012 starters allowed 528 ER in 880 IP.

(

Minnesota Twins Team Stats » 2012 » Starters » Standard Statistics | FanGraphs Baseball)

If we think they can be league-average in 2013, the ERA for the group will be around 4.10.

I assumed the improved performance would lead to longer starts, for 940 IP. The 4.10 ERA at that many IP is 428 runs.

So, 528-428... 100 fewer runs allowed.