This is a 3-4-5 right triangle. You can use any unit of measure you like as long as the ratio is 3-4-5 of that unit you can build a right angle using a straight edge and dividers. If you think in units only it does not matter which measuring system you use. I'm a Patternmaker with a small hobby foundry at home to make my castings for me. I do a lot of layout work using metric, inch decimal and inch fractional. An a lot of layout work can be done without a rule. Examples would be location of stiffeners on a flange, dividing a known length into any number of equally spaced divisions. This triangle is just the basics. Thinking in units of measure rather than mm, decimals or fractions, for me, makes working in any cad program easier. Still working on a lesson plan. I added 2 .stl files to show how each model looks in 3d printed form.
Print settings can be default. There is no stress of the part. I would use PLA and a .2 layer height.