Measure, mark, place fence, measure again before you cut. E.g. stick your circular saw on the piece like you're about to start, mark the blade position, check with a tape measure before pulling the trigger and adjust fence to correct. Check again after adjusting.
With just a circular saw for cuts like that measure distance from edge of blade (remember, blade is not infinitely thin) to edge of guide, draw a line with a square where you want to cut, move over by the blade-to-guide distance you just measured and draw another line. Clamp a long straight board along that line as a fence and ride the guide up the fence with the circular saw, double checking that the blade edge is at your cut line before you start. If you want, use a framing square to double check that your fence is clamped at 90 degrees before cutting.
Also if you have 48" and want 45, measure 45, don't measure 3 and try to cut it off.
If your stock isn't square trim both sides like this, squaring each off the same edge. Always pick one edge as a reference edge and square the other 3 off that. Use a straightedge to check your reference edge first, if you want, and if it's not straight, trim it straight.
You might find this guide useful. Describes a really simple and useful circular saw guide. Nice thing about that one is it shows you exactly where the blade edge is. If you switch blades to one with a different kerf you might want to make another base (narrower) or run the saw over the guide once after changing (wider) for an accurate edge.
Also if you're using a tape measure start from the 1" mark instead of the floppy metal end and remember to subtract 1 when reading. That's not your 1/4" but it can sometimes be 1/32" or so, esp. for cheaper tape measures.
You could use your table saw if it has enough capacity but I think a circular saw is an easier tool to work with for this kind of stuff.