Thankyou very much for all the info guys, the inspiration & release is just what I needed. With a first time build you're never quite sure what you can get away with. I ended up going larger & shifted the position further toward the ends a little.
As many have said, it is a bit nerve racking making the cut. I didn't want to use a power tool so I made up my own hand tool by using an 18TPI hacksaw blade & tapering down the end so I could get around the curves, it worked great with a nice thin cut & didn't take that long.
Glen G.