Given your intended itinerary (Umbria/Tuscany then Paris) from SFO the best option would be to fly AirFrance which has 1 daily non stop flight from SFO to Paris CDG (two daily flights in summer).
AirFrance also has 6 daily pairs from CDG to Florence (FLR).
Therefore the best would be to go to www.kayak.com and select a multicity flight as follows:
DATE 1 (date of departure): SFO > FLR: Many choices will come up with various airlines and routing. But the most convenient will be the AirFrance option with one layover only in Paris (SFO > CDG > FLR). That is your fastest route to Florence (about 14 hours in total).
DATE 2 (transfer to Paris): FLR > CDG (the AirFrance option will come up as one of the top choices)
DATE 3 (return to US): CDG > SFO. There will be various choices, but the non Stop with AirFrance will be the best for you.
One additional point. Citta' di Castello is approximately 1h45min drive from Florence, which would be the closest airport with connections to Paris. However Bologna airport (BLQ) is also not much further using the E45+A14 freeways (just over 2 hours). Bologna (BLQ) also has AirFrance flights to CDG, therefore you might want to try and replace FLR with BLQ to test which airport has the best pricing. Rome airport (FCO) would have even more flights, but it's a longer drive (almost 3 hours). Pisa is also another airport in Tuscany, but it's just as distant as Rome from Citta di castello, therefore you might as well go to Rome. I wouldn't bother with Milan (LIN) at all (AirFrance flies to Milan-Linate only), as Milan is well over 4 hours drive.