warning prometheus alert for rabbitmq queue space left

Can you help me with Prometheus expression alert that can monitor the space left of a specific rabbitmq queue?

For example, I have couple of rabbitmq queues with “x-max-length-bytes”= 115747500 bytes”:

   # rabbitmqctl -p v-host list_queues name arguments | grep 115747500
    queue-test1    [{"x-max-length-bytes",115747500 }]
    queue-test2    [{"x-dead-letter-exchange",[]},{"x-message-ttl",604800000},{"x-max-length-bytes",115747500 }]
    queue-test3    [{"x-max-length-bytes",115747500 }]
    queue-test4    [{"x-dead-letter-exchange",[]},{"x-message-ttl",604800000},{"x-max-length-bytes",115747500 }]
    queue-test5    [{"x-max-length-bytes",115747500 }]
    queue-test6    [{"x-dead-letter-exchange",[]},{"x-message-ttl",604800000},{"x-max-length-bytes",115747500 }]




    # rabbitmqctl -p v-host list_queues

      test1   471
      test2   4163
      test3   0
      test4   2466
      test5   0
      test6   0

I found on the google that the max_message_size for Rabbitmq is 32768 bytes, so I thought if I can have the below math expression:

(max_message_size(32768 bytes) * number queue messages) - x-max-length-bytes ( 115747500 bytes) = total_space_left_in_queue_bytes 
 if total_space_left_in_queue_bytes > 90% = send warning