I think it is a NERP calculation (New Estimated Run Production). About 8 to 10 years ago there was a quite extensive discussion in the Strategy section about it--I will see if I cand find it and put a link here.
This is the article I remember, but it includes the negative aspect (offense allowed) of the player from fielding also. The NERP formula is provided near the top of the article.
http://www.mfooz.com/bblog/wp-content/u ... efense.pdfSearching for "nerp" in the front page of this index (not in strategy section) will give some of the past links of discussions here.
One thing I will say is Diamond dope calculation is not the same as the ratings in the strategy guide, but it is derived from those ratings and still a useful way of comparing two hitters in a given park--