Something like 1:5, or even thinner, I would think.
Add a drop of detergent to break the surface tension and help it wash into the crevices, and cover the whole outside of the model...the idea being the faint brown wash will help the edges of the camou colours blend into the base colour.
As a bonus it will help pick out your rivets, and get into the wheel spokes...where it looks like there might be some unpainted spots?