Hey fellow PokerStrategists :)

I have problem understanding one thing in 'How to calculate pot sized raise' article: http://www.pokerstrategy.com/strategy/bss/1586/1/

In section 'Raising to pot size' example 2: if we use the basic formula, why is not our bet included there? It says 3x opponent's bet + pot before opponent's bet was placed. Before his minraise:

$15 were already in the pot + our bet $12 = $27

so we should be raising

3x$24 + $27 = $99 after opponent's minraise due the to formula.

But we raise to $87 instead. I don't understand why our bet is ignored in this calculation. Could anybody explain it to me? Thanks a bunch