Calculator? Didn't know one existied til now. My Google-foo has brought me this;
Never used it so.....caveat emptor, use the farce, and all that.
I have however, done the long hand via Moshers BT write-up. By far the best resource I have yet to see on the details of PG brewing. It, is here;
BT - Parti-Gyle Brewing
And yes, regardless of style, what you are proposing is a parti-gyle. A first running high grav and a second running medium grav. If you've never played with first wort hopping, you might take a look at that too to maximize hop utilization with respect to aroma and flavor.