I should expand a little.
As I also couldn't find any guides that matched how to do this style of buttplate, where, due to the curved buttplate, you can't go straight down and you can't start from the back either, I asked Ron Scott for advice.
His methods involve started from the tip of the finial, having the buttplate tilted so as you work your way down, it 'swings' into place.
This seems (sofar) to be a relative safe method, but being inexperienced like I am, I'm very reluctant to do any bulk removal of material, making progress very slow.
I have, deliberately, not inlet the tip all way, leaving ~1mm or so to do the final fitting once I'm close.
I'm now also rough shaping the buttstock as it goes deeper, making it easier for me to ensure that I'm not expanding the gap as I push deeper..
There's virtually zero consistent angles or reference point on this buttplate, and curvature and draft angles are constantly changing.
It's a little easier now that I've got contact on all areas, but it's definitely been a steep learning curve.
I'll try to make some drawings to better illustrate the process when I get home.