I just added a post clarifying seldom-understood details about this aspect of backstroke to a thread about how to improve SPL from 20 to the lower teens. You can read my post here
To improve SPL AND tune up the kick I would recommend trying an Asymmetric Tempo Pyramid such as the following
10 x 50 changing tempo each 50. Count strokes.
#1 @ 1.30
#2 @ 1.36 (slow Tempo by .06--six presses on right button)
#3 @ 1.42
#4 @ 1.48
#5 @ 1.45 (increase Tempo by .03--three presses on left button)
#6 @ 1.42
#7 @ 1.39
#8 @ 1.36
#9 @ 1.33
#10 @ 1.30
I recommend a smaller Tempo 'delta' for Backstroke than for Freestyle (you could also experiment with slowing by .04 and increasing by .02 or even half that -- .02/.01) because Backstroke won't support as wide a range of Tempo as freestyle.
As you slow Tempo, you should find stroke lengthening and the kick becoming more essential to the overall stroke--i.e. more integrated, rather than harder. As Tempo increases, strive to keep the sense of integration and Stroke Length.