if a is less than b then subtract a from b else subtract b from a.

AppleScript is an English-like language used to write script files that automate the actions of the computer and the applications that run on it.

tell application "Finder"
    set the percent_free to ¬
        (((the free space of the startup disk) / ¬
            (the capacity of the startup disk)) * 100) div 1
end tell

if the percent_free is less than 10 then
    tell application (path to frontmost application as text)
        display dialog "The startup disk has only " & the percent_free & ¬
            " percent of its capacity available." & return & return & ¬
            "Should this script continue?" with icon 1
    end tell
end if