Yes I just used a chainsaw. I used a framing square to mark a line then cut a 2 x 4 and used that as a guide. I measured from the new "bottom" of the stump to the proper height, in my case this is 17", and made a parallel cut. I checked it for square with a level and was right on the money. I think it took me about 5 minutes.
Trig doesn't scare me, it has nothing to do with what kind of word it is. If I were engineering something I would use it. I did for many years as a civil engineering draftsman and designer. I heard a very telling story once about an engineer and a blacksmith. The engineer wanted a one inch hole in a piece of plate at a certain point. He was using lots of math and figuring how to get a drill over the hole and how to make it all work out. The blacksmith asked how far from two edges he needed the hole. The engineer told him the dimensions of the hole and the blacksmith marked them out. Taking a torch, he cut a hole at the location leaving it just a little bit small. Heating the edges with the torch he then took a one inch drift and drove it through the hole. The engineer looked at the blacksmith and then walked away. For me, this little tail, true or not, tells me that practical experience should not be discounted. Nor should should the power of keeping processes simple. Over thinking a project can doom it to cost over runs and stagnation just as much as under thinking it. Your milage may vary.