Sometimes you will be faced with the challenge of fitting assembly to another (a molding to a carcase for instance). In cases like this it is best to use one piece to fit the other; the fancy term for this behavior being “verify in field”.
In the perfect world both your molding and your carcase would be square and you could easily do this with a knife. But you will often have little gaps from being out square. These gaps make it difficult to use a layout knife with any accuracy.
You could put yourself through the misery of squaring the exterior carcase to get things to fit flat, but I have a different solution. Get a bigger knife. Here I am using a plane blade. The large bearing surface gives much better registration to the carcase allowing me to mark a very solid baseline for my dovetails.
You can also press the mating board against the carcase to strike a second line. This line will be the length of your molding.
Now, go have some fun.
-- Make furniture that lasts as long as the tree - Ryan