If you have a swamped barrel, you might consider using the natural flare of the breech end of the top flat of the barrel as a guide.
In other words, make the front end of the tang the same width as the top flat, and then if you imagine extending the flare of the top barrel flat along the tang (in a straight line).
If your barrel is not overly swamped, this will result in a slight flare of the tang. Not too much, as Mike mentioned.
If it still looks too wide, then you can still file it narrower and reduce the flare.
I found it difficult to tell from your photo just how wide it is, but seems quite wide given the width of the stock.
Cheers,
Norm.