One simple tool that helped me when I first started reading them is that if you multiply the amount of cut or fill by whatever slope they want, it will always give you the distance to the toe of the slope. For example if your cut stake reads C 5.0 @ 3:1, then the toe of the slope is 15 feet from the stake. Remember though, that it is a straight line out from the stake and not 15 feet laying down the slope. You can't lay your grade rod on the slope and measure 15 feet out, it won't work. You can use your rod and take it in 5 foot increments to measure it out.
This tool really helps when you are working an odd slope, like say, a 2.6:1 or something.
Hope that helps!