This is a super useful tool for marking fin positions and other things on body tubes. It is very easy to use and produces excellent results. It is certainly accurate enough for building model rockets and a lot better than the "doorframe" method. The pencil holder hole is a bit small. It DOES say in the instructions that you may need to sand the pencil to make it fit. Being an engineer I just drilled it out by hand with a 19/64" bit to give a nice snug fit for my pencils. Six sided pencils fit best. Soft pencils make it easier to put marks on paper tubes. Overall this is a versatile and helpful set that makes building rockets much, much easier. Definitely a "must have" item for rocket builders.