I don't know. It has never come to my mind to try. For me it's an "one or the other"-type decission.
What is "normal"? The idle drain heavily depends on your setup, i.e. which bg active apps are installed and allowed to keep the device active? It also depends on curtent degradation of the battery itself. Idle drain for my setup with a three years old battery is about 0.6%/h. If I use whatsapp-web it's 0.8%/h.
Just dirty flash the rom and all boot partition altering packages, or use twrp's backup/restore, or make your own image backup with dd.