unable to add the dimension

[ben_reade]

Hello,
I am unable to add the dimension I want in my drawing. 
The dimension is from the top of the model to the bottom, as shown in this picture.

Can anyone help me?

Thanks,
Ben

[ClintBrown3D · Accepted solution]

When you are creating a dimension, if you right click, you will see a set of "Snaps", the top of the gearbox can be selected by choosing a "quadrant"