sed - adding a line when it doesn't exist

I have a file formsweb.cfg which might look a little like this

# These match variables (e.g. %form%) in the baseHTML file. Their values
# may be overridden by specifying them in the URL query string
# (e.g. "http://myhost.example.com/forms/frmservlet?form=myform&width=700")
# or by overriding them in a specific, named configuration (see below)
[default]
# Default Oracle Forms values:
#baseHTML=base.htm
#baseHTMLjpi=basejpi.htm
width=1366
height=734

[sepwin]
separateFrame=True
lookandfeel=Generic

[debug]
serverURL=/forms/lservlet/debug

This file is missing the key-value clientDPI=96 which should be located in the [default] section of this file.

On 100+ sites, some of the files might have clientDPI set, others might not.

I want to add the value to files which do not currently have it.

This is the sed script that I wrote. Is there a "better" way to do it?

sed -n '/\[default\]/,/^\[/ {
  /^clientDPI/q1
}' formsweb.cfg &&

sed '/\[default\]/,/^\[/ {
 /^\[default\]/ {
   a\
clientDPI=96
 }
}' -i.bak.${RANDOM} formsweb.cfg

It checks if clientDPI exists between [default] and the next [ character. If it does it quits with 1, otherwise, it goes to the next sed line, which does the insert.

GNU sed version 4.1.5