Thanks again. On to the green blockouts.....
To locate the correct spot for the b-outs, I first had to lay out a temporary decal placement which I did by simply using representative bits of cut paper. This showed me a small issue which bothered me. If you look at the arrow where the "B" would go, the dark brown colour intersects the B whereas in the photo of the actual aircraft above, the B is just touching the brown at the lower left corner.
I decided to remedy this by masking and adjusting the demarcation as shown below.
With the Middle Stone colour extended accordingly, I masked the forward edge of the blockout using the paper bit as a locator.
From there, I masked the blockouts to a 5mm width using a 4mm letter height as a basis. Dark Green was then sprayed both sides after making sure that sufficient masking was applied to prevent any overspray from marring the finish underneath.
Pealing the tape away revealed the effect I wanted. I had to do a small repair on the port side where I chipped some of the brown at the upper right corner of the green blockout but, overall, it turned out allright.
Now comes the gloss coat and decal application. Thanks once again to everyone for your comments.