Set relative to progress bar

I am making a game with achievements and a progress bar to track those achievements. But I can't set relative to progress bar, only absolute. I tried setting a variable and getting the progress off that but it won't take the variable as input.

Please help!

Thanks!

say your bar is at maximum 200 pixels wide, maximum achievement points = 500, and you currently have 5 points.

bar.maxwidth = 200

bar.maxpoints = 500

bar.currentpoints = 5

bar set bar.width = bar.maxwidth * (bar.currentpoints/bar.maxpoints)

would get you a 2 pixels wide bar.

bar.currentpoints = 300

would get you a 120 pixels wide bar.

Thanks so much!