Recent estimate of yet-to-find impact craters on Earth

A recent attempt to estimate the number of impact craters 
remaining to be found is:

Stewart, S. A., 2011, Estimates of yet-to-find impact crater 
population on Earth. Journal of the Geological Society.
vol. 168, no. 1, pp. 1-14.
http://jgs.lyellcollection.org/content/168/1/1.abstract

He estimates that there are about 845 impact craters larger 
than 1 km in diameter that have been preserved over the 
Phanerozoic. Of these, he estimates that about 714 craters, 
of which 228 craters are larger than 2.5 km, are yet to be found.

Yours,

Paul H.