No question is 'too embarrassing' to ask if you don't know the answer. We have all been in that situation. Even after many years as a quilter there are lots of questions I have. When one becomes relevant I ask. Triangles are perplexing. Recently, I struggled with directions for using the ruler for Hunter's Star. I just was struggling with it. I took it to a local quilt store and asked for help. It was freely given. My problem was putting triangles together with a trapezoid. Once shown it made perfect sense and now I am on to the next step. Like others, I do believe the measurement you are seeking is for the straight sides rather than the diagonals so that the finished unit (square) has the measurement you require. Another example is the triangle made with the tri-recs tool. When choosing patterns just be sure you understand the directions when you read them. Some pattern writers are much better than others.