I'm working on an image of the layout of a building floor plan. Let's say one of the rooms is 25 feet I need to add 8 feet onto this room. So how do I set my ruler tool to the size I know & have it calculate the new size? I hope this makes sense.
Look at the image, set a couple guides to mark your distance (a start and end distance). In this case, a known distance of say 1 foot or 5 feet or 10 feet or 25. I'm assuming the distance will be either horizontal or vertical instead of diagonal (diagonal is fine, but the flow for this one will need to change). Turn snap on (View >> Snap should have a check).
Switch to your Ruler tool (the letter I or go to the tools palette and find your ruler with the eyedropper tools). Make sure you have the Info palette up, too (in Palette Options, change mouse coordinates to pixels). Now, hold down Shift, position the ruler crosshairs on the first guide, click and hold with the mouse and drag to the second guide. Snap should help with getting the guides just right. Now look at the Info palette and it will tell you # pixels for the distance you just measured.
We're figuring pixels per foot here (math part):
number of pixels in the measurement / number of feet in the image = pixels per foot.
To figure the number of pixels for 8 feet, multiply 8 x your pixels per foot above to get the number of pixels to add (say, in the Canvas window).
To make things easier, you can do a little conversion on the image first:
For example, if the image is 300 dpi and you have 3 feet per 2 inch (or 600 pixels per 3 feet), that's 200 pixels per foot. To make 1 inch into 1 foot, convert the image in the Image Size palette to 200 dpi, but deselect Resample image. There won't be any change in the number of pixels, but your inches will increase. When you're done, do the conversion back to 300. This way, when you add a foot, you go to the canvas tool and add just 1 inch, you also add a foot to the drawing (cuz 1 inch = 200 pixels = 1 foot).