I hate to say it but I've never really seen such a resource. I usually use the little "Description" field in BeerSmith, it was pre-populated with some really good information, including "recommended percentage of grist" for each type of grain. Certain grains even say, for example, "3%-10% for stouts, 1%-4% for brown or amber ales."
Some - but not all - of the info is on individual info pages for each grain. For example, BSG posts rates for grains by manufacturer, here's Briess: http://www.brewerssupplygroup.com/malt/briess.php
You can see that Pale or Wheat are OK to 100% but they recommend that Black Malt consist of 5% of the grainbill or less.
I think some of the numbers are a little vague. But it's a start. I've used this page a handful of times in formulating.