In necksizing my '06 brass, I generally size about 2/3 of the neck--mas o menos, give or take a dollar 3.80 a pound or so...It doesn't seem to matter, so long as 1/2 or more of the neck is resized...
Never having messed with Herter's dies, I'm guessing that there is the usual threaded hole in the center? For the decapping/expander rod? If so, and you're lucky, the diameter and thread of the Herter die might fit the rod from your RCBS die...
Also, if you run the case only partway up into a full-length resizing die, so that only around 1/2 or so of the neck of the case is resized, the main body of the case is not resized...
Good luck, Art