Making a square - Joining two pieces of wood cut at 45 degrees with a circular saw
You need to adjust your circular saw to an actual 45 degree angle, not to where the scale markings say '45'. Your speed square will do the trick.

(Or redesign for butt joints and skip the bother of a circular saw for 45s like that...)

edit to add, I tend to avoid cuts like this with a handheld circular saw because they are hard to do well, but you can improve the cut by concentrating on keeping the large part of the shoe (aka sole/deck) flat on the wood, and using something like your speed square to guide the saw to a smooth 90 degree cut.