The computation of 50%90% don't work as expected.

The computation of 50%90% don't work as traditional maner.(Cohn, Mike. (2006). Agile Estimating and Planning. pages 194-196 . Prentice Hall).

How to recreate:

 - Get the example corporate website backlog

 - Activate 50/90 for this project.

 - Add 20 points to INR1 and 20 more to INR3

Theme weight is computed as 36.6 (15+21.6?) points while whith 50/90 it will be 15 standard points and a buffer of 5.47  = 20.47

Thank You