Essentially, without knowing it, you re-invented the wheel.
As I understand it, long long ago, before there was such a thing as a laser, they used "batter boards" which were basically the same thing. They'd pull a string, set to match the required grade, and then measure down from it.
If you want to get really primitive, you could get a ten foot 2X$ and a level, and check the amount of fall in the lenght of the board. The problem with that is you can compound your error too quickly--if you're 2 inches1 deep and you make that same mistake three times in a row, now you're 6 incehs deep.
You could also set up a plain old transit or builder's level and figure out how much fall you'd have per foot of ditch, and then take sights at known distances from where you began. That would keep you on grade too.
Then, of course, there are lasers these days.
On edit: Watching the bucket pins was a good idea too. If I'm digging something that has to be a constant depth below grade, like a gas or electric line, rather than measuring constantly, I'll just note how that depth compares to the bucket pins, and dig to that depth.
