elastic yarn and amount needed

[dicon77]

I'm planning on making a tank top for my daughter. Recommended yarn is Cascade Pima Tencel 6 balls (109 yds/50-g ball).

My daughter likes Cascade Fixation Effects, which is 100 yd/50 g relaxed, 186 stretched.

Yarn.com has 5 balls of the Fixation. Would 5 be enough, meaning do we take into account the "186 yds stretched," or do we ignore that, only go by the 100 yd/50 g and still buy 6 balls?

Jodi