The focal length of the mirror will be fairly close (+/- about 1%) and the actual focal length of the various actual mirrors will vary on a long term production basis. The maker does attempt to keep them fairly consistent as his testing equipment is pretty much all bolted down so he can do the tests real fast. Also, the grinding equipment is maintained to keep that advertised focal length where it is listed as.
You still do want to bolt the mirror cell to the tube as the last part after you have insured that the focal length is correct. You want to take a 25mm EP (regular Meade or Celestron cheapie) and make it focus about 3/4th of the way out on a typical focuser and that should allow for the very short 4mm EPs to focus also. If you have a 40mm Celestron 1.25" EP then use that almost all the way out on the focuser as there is nothing that wants a further out focus point.