One of the most frustrating things is, if you're copying an existing cab, don't be surprised if the numbers don't add up. Look at the numbers, the part under the marque is probably more than the 15mm written on the pdf. Maybe a bad scan and should read 75mm?
Anyway, the angles I measure with the tools I have are usually in the +/- range of 5 degrees, if good, and +/- or 10 if "close".
Other than that, good luck, and if you need any help with sketchup, we can help.