Strange, because Larry Coon's CBA FAQ says something completely different:
http://www.cbafaq.com/salarycap.htm#Q97
I stole your roster: TP for typing it out so neat.
2012-13 Boston Celtics
PG: Rondo ($11.0), Terry ($5.0?)
SG: Bradley ($1.63), Lee ($4.2?)
SF: Pierce ($16.80), Green ($8.0?), Joseph ($0.48)*
PF: Bass ($6.5?), Wilcox ($1.35), Sullinger ($1.3)
C: Garnett ($10.5?), Melo ($1.25)
Total: ~$68.1 mil
TBD: Dooling, Pietrus, Stiemsma, Daniels, Sasha
So you got 12. You sign Dooling and Stiemsma for depth to get to 14. For 15, you could go with Pietrus or Sasha to be the 3rd string SF. This player may be able to get minutes because the backup SF, Green, can play some at the 4. Also, with Bradley missing time early, this player could also get a few SG minutes.
My question is, why not Delfino over Pietrus? Is Pietrus superior enough on defense that he would fit better? The talk about Delfino got me interested in him. Use the BAE on him and then Stiemsma is out of the picture?