Getting your shots leveled for architecture,.. aargh

I am getting more and more into shooting buildings. and getting somewhat frustrated too 

I find it very hard to get right in photoshop (and I am very experienced with photoshop) so the best route is to get it right in the field, as always.

Using the built in level of the D800 just doesn't cut it. Once you have it set you can still move the camera a bit left or right and it will still tell you the shot is level. Still much better than doing this by eye.

Next is the built in level in my tripodhead,.. somehow it's not level?? how can a level be off?

Last I have the little hotshoe level and even with that I could use some improvement.

Today I put a carpenters level on top of my camera and that has a much bigger scale and therefor more precise.... but not really mountable and not nice to lug around in the field.

Last on the list is the Iphone, it has a nice app which does 0.1 degree steps.

So it would be very nice to get an iphone mounting device this would give the best results.

The only problem there is that you need to calibrate the iphone and for that you would need a surface that is perfectly leveled... aaaaarg 

Now if only the D800 would show those minimal steps of 0.1 degrees... I am sure they can?

any suggestions tips from the pro's are more than welcome, I am pulling my (last) hairs here