If you are going to stick with the original style setup, then I would list the OAL according to Heckler and Koch. Since you said this would be an SBR, I'm assuming you mean the MP5K-PDW (folding stock) and not the regular MP5k (front and rear pistol grips, but no stock), and so you should be at 22.44 inches.
If the gun in question does not have a 3-Lug barrel and the barrel terminates at the triple tree, then things get a little tricky, because your gun will be shorter than the published specs for the MP5K-PDW.
I don't know how much of a big deal the accuracy of the measurements really is. My M92 SBR is papered at the incorrect length. When I filed for it, nobody had performed my exact conversion yet, so I couldn't get any help from other owners. Best I could do was make an estimate based on published numbers, which turned out to be off by about 2 inches. Published specs being that far off, who woulda thunk it?
What does the ATF want me to do, buy all the parts and measure it out myself before filing? Don't they also consider that to be "Constructive Posession?" The soothsayer aspect of forecasting the exact length seems to be some sort of cruel joke.
At the end of the day, the ATF knows you are in possession of that firearm in an SBR format. Do they care if it is one inch or two inches off? I don't know. I understand that they like to be informed of changes, but that's where the train tracks end.
