Acrobatics for moving through threatened squares.

[P_Rook_99]

Hello!

In a game I hosted today, a player wanted to do the following:

Move 10 feet towards a fire elemental, using his normal speed, and then, using his remaining 20 feet and halving that, he would use his acrobatics skill to move through threatened squares.

My first thought was that he could either move 15 feet at halved speed, or move 30 at full but at a higher DC, but not suddenly half your remaining squares.

What are your thoughts on this?

(http://www.d20pfsrd.com/skills/acrobatics)

[SlimGauge]

My group only counts double movement costs in squares where you are actually moving acrobatically, not in every square of the entire move action.