How difficult the buttplate is depends on the accuracy of the guy precarving it, and also the style of the buttplate, and how precise you want it to fit.
For me, a Acanthus Leaf is about the hardest one I do.
In another direction, if you would have made a Scribe Line at the breechface on the barrel flat, you would know Exactly where the face is. Using a felt marker leaves a variance, as you don't know if the line is for , center, or aft the marker line. I scribe mine with the back side of the tip of a Exacto knife. 1 thin distinct line. I suggest you check again with the RR & find the breech face & be sure & precise on the line.
Then you KNOW where the breechplug face is Exactly.
If it is where you want it, then you are ready to do the Tang. Bend it to shape, bevel it, inlet it. As you inlet the tang, they generally try to push the barrel forward & you must relieve it as you go to keep the barrel all the way back to the stock at the breech.
Get the tang in & down & shaped to what you want, with no pressure holding it in place. Then pin the barrel in.
Keith Lisle