I'm posting a cheat sheet that I created for FasTrack lengths. It lists all combinations of FasTrack pieces up to 25 inches total length.
Some lengths require you to remove an adjacent 10 inch piece and replace it with a combination of pieces to achieve your total length. For example, if you need a 3 inch piece there is no combination of track that provides it. So, in this case you would need to remove an adjacent 10 inch piece and create a 13 inch combination (10 + 3 = 13). A combination of a 5 inch, 4.5 inch, and 2 1.75 inch pieces will provide a 13 inch total length piece.
If you are using AnyRail software for track layouts you can insert a ruler on your plan. If you magnify the plan it is easy to figure out the length you need by manipulating the ruler. Perhaps other track layout software provides the same feature, I don't know.
Hope this is useful...
Larry