You might be looking for a scripted metric aggregation here, which is not supported in Kibana at this time.
To operate on ratios (which is a common use case), I'd suggest a couple of approaches:
- At the current time, see if you can index the ratio directly as a field, either at data ingest time using Logstash or after the fact using Watcher
- Wait until Elasticsearch 2.0 and a subsequent Kibana release that adds support for a bucket script aggregation, which will be able to do arithmetic on results of aggregations.