The magic number for right angles and 45 degrees is 1.4142. Just remember 'forty one, forty two'. I use it all the time at work for quickly working out rough areas off drawings (at least I did until I discovered you can pull an area off a pdf drawing straight out of the software), and for setting-out.
How you use it is simple. On a 45 degree isosceles triangle (One right angle and two 45's) if you have either short side a size of 1, the hypotenuse will be 1.4142.
So, stick a DI on the top slide and dead square/touching to the tailstock ram, wind the top slide 141.4 thou and adjust the angle until winding it in 141 thou moves the dial 100 thou. You can use any multiplier to gain accuracy.