sizing parts of an object

I am wondering if there is any way to scale an object so that a section of it is a specific size...


ie. I have a raster image which includes a black outline and extra white space. I have drawn a shape over the top of the image just inside the black outline. I now need to make the shape 100mm wide, and keep the bitmap the same ratio.
If I scale them both at the same time then I only get to type in the size for the bitmap, so to get the shape the right size I need to scale with trial and error.

It strikes me as something which people must constantly be having to do, and everyone must be having to simply guess until it is reasonably close... which isn't exactly a very professional way of doing it.

So, is there any way to specify the part of the image I am sizing? eg, will it let me select the two further most points of the shape and tell it to scale them to 100mm, and it scale the bitmap to the appropriate size?

Find out the dimensions of the shape you have drawn on top of it. Then figure out the percentage it needs to be scaled to make it 100mm wide. Then select the raster image and the shape you have drawn and scale them both together at the determined percentage.

I have found Scaletron! invaluable for helping me figure out scale percentages. You can download it for free here: http://www.rocketjam.com/scaletron.html