Hello sudi,
For me you have 2 options :
You have the logstash solution (do the multiplication by logstash) : you should use the Ruby plugin as recommended by @magnusbaeck on this topic.
Or (it's better for me), try to use Kibana visualisation to do the operation, it's more flexible to get raw data and manipulate it trough Kibana (you may use in the future).
Hope it's help you !