years of documentation / testimony here to read
Any largely wind-impermeaable - hammock will, with a well-suspended 20F UQ, take you to 20F, assuming you are wearing fleece, wool socks, a hat and/or balaclava, and maybe a thin sweater. Some here, including me, can use just a 30-40F quilt on top. Others would want a 20F rated TQe or equivalent bag.
Wind changes that, just as on the ground. So hang your tarp to block the wind.
Don't want to go integrated "weather shield' or equivalent, what with zippers and expense? Hand or machine sew some fabric snaps onto your hmmck side hems to match up to the other halves of those snaps on 2-3 yd^2 of wind-shield fabric. Nylon, poly, fleece, whatever you like. Cost? $10-$20 for a weather-shield
I have (too) many Clarks, and I'm not getting rid of any, most with weather-shields.
Q:10F of warmth added by ANY weather-shield MARGINAL to being well tucked in without one?
A: If it makes people happy to think so, let them think it. I deploy mine all the time because the marginal cost of doing so is trivial, and protection from BIG DRAFT is substantial, should gusts come through with a weather front, or should I un-tuck my TQ by accident.
But, I'll tell you that probably nobody here who has slept outside at 20F honestly thought on the FIRST experience that he / she would get used to breathing such cold air and sleeping, too. Turns out that just like exercising in heat, there is some adaptation to non-normal temps, and no real short-cut to it.