Since we can't manipulate the Start/End of the color ramp directly with a parameter what we need to do is go one step upstream and change the *measure* that is used for Color such that instead of it's top end being the max value in the data it's the top end parameter. So the original measure will still be used on the Text shelf and the new measure will be used on the Color shelf.
Here's a view built using CoffeeChain sample data:
And here's the Color measure:
IF [Use Top End Parameter] THEN
MIN(SUM([Sales]), [Top End of Sales for Color])
This is using the feature of the MIN() function that will return the lower of two given values.
I've attached a v9.2 workbook.
set end color.twbx 142.5 KB