New Scaling tool.

I've tried the new scaling tool that showed up in my toolbox. I've been waiting for years for something like this to be added. It seems to be very inaccurate. I put in the correct scaling ratio, and when I click on the line it is off quite a bit. With imperial, general architecture or landscape scale is one: 20. That means 1 inch equals 20 feet. The actual ratio is 1:240 I use that in the dialogue box and a 40 inch line which should be 800 feet came up at 778 feet. I've tried it a number of ways and it is always wrong. Is there another adjustment?