Level Transit

I created this Level Transit to help me while digging some drainage. My hope is to use this to keep me from digging a ditch too deep. The idea is that since I know where I want the ditch, I don't need to use any of the angles. I just need a level sight and a measuring stick to maintain a depth or a grade. Yes, I know it can be done with a laser and receiver, but they are expensive.

I plan to use an old tape me