Doesn't your tape measure have both inches and millimeters on it. Mine also has a Planck length scale. <img src="http://www.catsailor.com/forums/images/graemlins/cool.gif" alt="" />

I few years ago I came across a drywaller's T-square (24"X48") and I have found in very handy for boat building.
At 8 dollars I think it's the least expensive tool I have.
The biggest problem with it is it does not have a metric scale on it. Next time I go to Canada I will have to get one there.