TVS1962,
I have an old edition of the ASD Manual of Steel Construction. Under Dimensions and Properties, it says, "American Standard channels (C) have a slope of approximately 16 2/3% (2 in 12") on their inner flange surfaces."
It also says, "The properties of these rolled shapes are based on the smallest theoretical size fillets produced; dimensions for detailing are based on the largest theoretical size fillets produced. These properties are either exact or slightly conservative for all producers who offer them."
Better grab a hold of the Manual. In there you will find "k" for a particular channel size. "k" is the distance from outer face if flange to web toe of fillet of rolled shape. For a C10x25, k = 1". The "T" dimension specified by COEngineeer is the difference between the depth (d) of the channel and 2 times "k" (i.e. for C10x25, 10"-2(1")=8" )