Critical threat range question

[Robai]

Feats and Enhancements bonuses usually stack (there are many examples).

Is it really true that Improved Critical (doubles the threat range) and Kensei Mastery III (+1 critical threat range) stack too?

For example, Sword of Shadow has critical threat range 18-20.

With Improved Critical feat it is 15-20.
What will happen I take also Kensei Mastery III?
15-20? (i.e. doesn't stack)
14-20? (doubled first, and then +1)
13-20? (+1 and then doubled)

I guess it is 14-20. Right?

[t0r012]

Correct it is 14-20
Use standard math multiply & divide then add and subtract.

[Tinco]

I guess it is 14-20. Right?

Congratulations, you have won 100 critical stacking points and a free vacation in the untouched natural wastes of the Mournland.

[Robai]

Correct it is 14-20

Thank you.

That means I will TR to fighter now