You are doing very well.
A measure yet to be confirmed is the present height @ 261.5 Rc (same as yours) with a 51.2d slope and a width of 4m at the apex.
From my calculations I'm betting on 266.7 Rc as the intended height.
Your 266.2Rc is right on !!!
It pays to be scrupulous...don't you think?
What do we do with all of those 274.67 Rc heights that everybody has used in their calculations?
What of the 4:3 seked ?
Oh....my....my...we have opened another can of worms !!
Life is not short....it is eternal.
It is we who have the short pleasure passing through it !!