Dynamic Server Weight from a variable

Hey folks,

im trying to configure server weights from environment variables. I ended up with this but its not working (503) (stripped away not relevant parts of the config)

global
  set-var proc.percentage_new int("${PERCENTAGE_NEW}")
  set-var proc.percentage_old int("${PERCENTAGE_OLD}")

backend percentage_strategy
  option tcp-check

  cookie sticky_server_cookie insert indirect

  # old domain
  cookie old_domain
  server old_domain_percentage example.com:443 check ssl verify none weight var(proc.percentage_old)

  # new domain
  cookie new_domain
  server new_domain_percentage new-example.com:443 check ssl verify none weight var(proc.percentage_new)

googling doesnt give my any hint if thats possible. In the end i want to create an immutable docker image which i can reconfigure just by providing the weighted env vars:

docker run -e PERCENTAGE_NEW=5 -e PERCENTAGE_OLD=95 my-haproxy

also i thought basic arithmetics are possible so i only have to provide the PERCENTAGE_NEW and do a simple calculation for the old weight like

server old_domain_percentage example.com:443 check ssl verify none weight 100-var(proc.percentage_new)

but that doesnt even produce a valid config. I didnt wanted to go the lua route, if its not possible its ok to provide both variables (the math upfront isnt that hard i guess )

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