A great thing to have on hand is a zero-clearance circular saw guide. You can make one (I have multiples in different lengths) with a couple of boards you can get at a big box stores, some glue and a few screws (optional).
Get a length of MDF board. Something like this:
You want this to be dead straight so you'll want to buy it in this form. I would go with the wider pieces but it depends on what you need. Don't get it too thick or it will limit the depth of your cut.
Then take a piece of thin plywood that is as long as your MDF board and at least as wide as the MDF plus the with of your saw base. Glue the MDF to the plywood like shown in the following picture from this article with instructions:
The article suggests different materials but the MDF plus plywood worked for me. Then take your saw and put the base up against the MDF fence and cut the wide side of the guide. Now you have a zero-tolerance jig that you can clamp down and know that your cut will be dead straight and exactly on the edge of the guide.
In your specific situation, I would mark the line you want and then take the guide and line up the edge of the guide parallel with your marking. Place the guide on the side of the piece you wish to keep if possible. If you are forced to put the guide on the waste side of the cut, make sure you consider the kerf of your blade. Clamp in place (I like quick-clamps for this) and you can't miss.