Hairpin crescendo/decrescendo

[Nick Payne]

I'm trying to reproduce a score that has quite a number of bars with a
crescendo in the first half of the bar and decrescendo in the second
half, and the hairpin is symmetrical left and right. In my example
below, the hairpins in the first bar start and finish at the correct
locations, but in the actual score I have to tweak the vertical position
of the decrescendo hairpin in each bar to get them to align
horizontally, in the second bar the hairpins don't match the original
score as they are of different lengths. Overriding Hairpin
#'bound-padding as in the third bar gets me close, but not exactly. Does
anyone have a suggestion on a better way to do it?

\version "2.16.0"

\relative c' {
  \mergeDifferentlyDottedOn
  \mergeDifferentlyHeadedOn
  \time 6/8
  <<
    {
      e8\< g b\! e\> b g'\! e,\< g b\> e b g'\! e,\< g
      \once\override Hairpin #'bound-padding = #6 b\> e b g'\!
    }
    \\
    { e,2. e e }
  >>
}

test.png
Description: PNG image