You can absolutely be comfortable with extra blankets stuffed inside the bag. I have done exactly this using a bag with the exact same ratings. It was juuuuust below freezing all weekend, my bag was rated for 20 degrees, and I was cozy with a few extra fleece blankets cocooned inside the bag. Maybe bring a wool blanket or two if you’re expecting it to be wet at all, because wool will retain heat even when wet.

Also, change your damned socks before bed. Lots of newbie campers make the mistake of going “I don’t wanna take my socks off and get my feet cold right before bed. I’ll change my socks when I wake up!” In reality, if you go to bed with socks that have an entire day’s worth of sweat and skin oils soaked up, your feet will stay cold all night. Change into a fresh pair before bed, so they actually insulate your feet instead.

Get a sleeping bag liner. A flannel one will cost about $30 and boost the warmth of your sleeping bag considerably IF you need it.

Pro tip: If you are cold in your bag when you shouldn’t be it probably means you got too hot, started sweating, and now your cold because of evaporation. It took me YEARS to figure that out. I now leave my bag unzipped half way down so I don’t build up moisture inside and that keeps me from getting cold. I’ve comfortably slept in my bag many time in air temperature well below freezing since figuring that out.