Late May a 20 degree bag would be suitable for me, barring a late severe storm/odd cold snap.

There are a bunch of ways to bump the temperature rating of bags.

1. Give a look at a vapor barrier liner. They are just a bag liner which does not breeth. It makes a "micro climate" inside, making your sweat not go throught the insulation and out of the bag. Some people don't like that they leave you clamy. Don't wear all your clothes inside the VBL. Thinweight longjons&top are all that is needed (if any). If you need more insulation, stuff clothing in your bag between the VBL and bag.

2. Wear your hat & mittens. Add other clothes as needed.

3. Take a look at the Finbar Sleep Hood. I have not trully field tested it, as I haven't had it long enough. It will increase your comfort rating 15 to 20 degrees (I'd imagine) at a 6 to 7 oz weight penalty. It's made by a Christian Brother, who is a very experienced outdoorsman. Note that he's a little hard of hearing, but very nice to chat with should you call in an order. I recieved mine 2 days after I placed my order.