Minor, but you don't include the "standard allowance" of 1/8". If you're trying to be exact to the 1/16th of an inch then an 1/8" is "huge"
You could have a dropdown for the allowance amount, i.e. 0 for exact, +1/16, +1/8 etc, as well as Sight (which adds considerable complexity...) and Outside Dimensions.
But, you might also want to clarify what "moulding width" means. It would appear from the formula that it is the width at the back of the moulding, not the face. That's the proper way as far as I'm concerned as you don't need to account for varying rabbet widths, but if people want to calculate to the nth amount they should probably know. And if you add in a Sight Dimension option you will need to have Rabbet Width, and in that case you could have Face Width instead of width at the back.
Additionally, in the realm of precision I guess you could also add a field for Kerf (inches and fractions) to allow for various types of cutting instruments.
My daughter recently went through this for our website (which will be updated soon). It's not for free-form entry though - it uses our moulding specs from our database, so all you need are the dimensions and specifications (Sight, +1/8, etc), and it only tells you what we will charge you, not actual inches - but all the same issues come up...
