Problem with the ‘IF’ command

Hi! I´m trying to make a resume of savings in a theoretically donation transaction but I´m been unable to do it.

The formula is: 1-250 saves 80%
and <250 is: first 250, 80% And the rest 40%

For X <= 250€: X*0.8
For X > 250€: (250*0.8) + (X-250)*0.4

Any idea how to construct the ‘IF’ to work properly and as expected? formula/sentence?

The page I need help with: [log in to see the link]

 

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