I've noticed the center of the bolt face on my High Power Service Rifle is eroded, right behind where the primer sits.  You can see it clear as day with a 10X Coddington hand lense.  I just removed a ring of carbon around it, actually.  The bolt has fired probably 3,500 to 4,000 rounds, and the erosion is no doubt my own fault for reloading old brass with worn out primer pockets.  In any case, the depth to the un-eroded outer half of the bolt face (from the front of the locking lugs) is 0.123".  The depth to inner part of the bolt face (where eroded) is 0.133", so the erosion depth is 0.010".  Is this enough to cause a loss of precision or other issues?  If not, at what point is it time to replace the bolt?  This a competition rifle, not a range blaster, and I shoot in the High Master class so shooting precision is my primary concern.

This.

In general it doesn't actually matter, if you are talking the primer diameter ring from gas leaks atound the primer.  That's in many ways the one spot on the bolt face that sees the least force on it.  Replace if it bothers you - bolts are cheap.  But really, I'd run it until just toss it after next barrel change needs another barrel (i.e.around 10,000 or so).  

Bigger question is correcting your ammunition gas leaks.  What load are you running and what primers?   Hot loads with CCI400?

Or am I miss reading, and it's a uniform step depression the size of an entire primer diameter?  That would be a new one to me, if so.

Congrats on shooting high-master, that's not easy to do

Match barrels only last about 3000-5000 rounds in high power before groups start opening up at 600 yards. Many shooters replace bolts every barrel or every other barrel because they run 600 yard loads pretty hot. Replace the bolt next barrel change.

If you are stretching pockets to get gas leakage your loads are nuclear hot or you have serious carbon buildup in the barrel. I have ran loads that stretched pockets but have never had problems with gas leakage damaging the bolt face.

You won’t be getting high master scores stretching pockets when a loose primer finds its way into the trigger. I have seen this happen several times on the line.