Extra Effort Costs for Psionic Powers - pls help

[Thökk]

Hey!

Page 7 of GURPS Psionic Powers says following:

"You must determine what level of power you’re trying to attain before rolling. Then, pay 2 FP and make an extra effort roll – this is a Will based skill roll, with a penalty of -1 for every 10% increase in level (or fraction thereof) that you are trying to attain."

"Example: ...with Telesend 4. She’s now convinced he’s out of her normal range. She decides to use extra effort to raise her Telesend (p. 60) to level 5, a 25% increase."

I don't get what it means by "10% increase in level". 10% of what? Point cost?
I don't get how it was calculated that extra effort from 4 to 5 here costs 25%.

Can somebody clarify this rule to me, please.

P.S. How do I calculate extra effort to get from 1 to 5? From 4 to 9? Can you walk me through it?