Right, seeing how Kit asked nicely I thought I better show him the tool I bought from Lidl.
So here is the tool....

Switched on.....

At an
angle
Reset to Zero which is useful to find the difference between two
angles
The angle between the last picture and this one...

And back to the start, well almost. 0.1
o out by me not the device.

And with the arms moving the other way.

Hope the pictures are informative. The tool comes with an instruction booklet, but really its so simple to use I haven't even looked at it.
Gondor