I can come close to the same SPL range I use for freestyle when swimming backstroke, but must swim with a VERY low SR to do so. Which is natural since my freestyle repeats are 5 to 8 seconds faster per 50.
I find that the more significant difference between my freestyle and backstroke is Stroke Rate. I was very surprised the first time I used a Tempo Trainer
in backstroke to find out how low my SR was. Therefore I wouldn't suggest trying to match your freestyle SPL. Instead find a reasonably efficient SPL for backstroke -- perhaps 2 to 4 added strokes in 25yd/m -- then work with the Tempo Trainer, in the same way as freestyle, to hold a particular stroke count while incrementally increasing tempo.