Bottom of the top hole, and top of the bottom hole!
It's not a perfect fit Rich, but with the measurements I gave you I'm not surprisedyou did outstandingly well to get it as close as you did!!
Bought myself an Aldi/Lidl digital measuring stick and the 19mm shroud is nearer to 19.2mm!
Ahh, well it's fitted and stops the floating barrel from floating!
Cheers.
Roy.