Kit instructions sometimes tell you to simply use a straight edge when cutting out fins or card stock. Simply setting a the strait edge along the cut line doesn't account for the thickness of the knife blade.
Here I'll be cutting a strait line between two marks in card stock. (You can certainly use this tip on balsa.)
Don't set the strait edge down yet.
First, set your knife blade right on the cut line. You can punch the tip into the card stock.
Leave the knife in the card stock line. Then slide the strait edge up to and against the knife blade.
Here you have made a slight space allowance for the thickness of the knife blade.
Adjust the strait edge position on the left until it matches the spacing on the right.
Now you can cut the entire line starting from the left and going to the right. You are guaranteed your cut is exactly on the cut line.