Create a copy of defaults.lua and name it defaults.persistent.lua
Then modify the tap zones in defaults.persistent.lua. I think this gives the zones what you want. I use something similar on my Kindle except I don't have a tap zone for previous page at all. I use swipe for that. Majority of the touch area is set to forward on mine.
Code:
-- customizable tap zones(rectangles)
-- x: x coordinate of top left corner in proportion of screen width
-- y: y coordinate of top left corner in proportion of screen height
-- w: width of tap zone in proportion of screen width
-- h: height of tap zone in proportion of screen height
DTAP_ZONE_MENU = {x = 1/3, y = 0, w = 1/3, h = 7/8}
DTAP_ZONE_CONFIG = {x = 1/3, y = 7/8, w = 1/3, h = 1/8}
DTAP_ZONE_MINIBAR = {x = 1/3, y = 31/32, w = 1/3, h = 1/32}
DTAP_ZONE_FORWARD = {x = 0, y = 1/8, w = 1, h = 3/4}
DTAP_ZONE_BACKWARD = {x = 0, y = 7/8, w = 1, h = 1/8}
DTAP_ZONE_BOOKMARK = {x = 7/8, y = 0, w = 1/8, h = 1/8}
DTAP_ZONE_FLIPPING = {x = 0, y = 0, w = 1/8, h = 1/8}
DDOUBLE_TAP_ZONE_NEXT_CHAPTER = {x = 6/8, y = 0, w = 2/8, h = 2/8}
DDOUBLE_TAP_ZONE_PREV_CHAPTER = {x = 0, y = 0, w = 2/8, h = 2/8}