Reducing width of back panel

I’ve made a Muslin. A large. I want it to sit on my natural waist. I sewed a size that should have resulted in 1/2 inch ease at natural waist but I’ve had to severely pull in elastic at back panel (41/2inches to be chopped off elastic). The front seems to fit reasonably well. So I’m thinking of reducing width of back panel. How much would people recommend reducing width of back panel by? Or I could go down a size and also reduce back panel but that might be playing with fire. Thanks Joan