Mathematics across measurements

Apologies if this is a duplicate, I had a look and couldn't see a relevant issue.

I can see from the documentation how to select from multiple measurements (although it calls them series, still, at https://influxdb.com/docs/v0.9/query_language/data_exploration.html )

For example, with data inserted by telegraf, you can do:
select * from disk_used,disk_total where host = 'myhostname' and path = '/'

How would you express that as a percentage? I've tried variations of the following, and none seem to work:

select disk_used.value/disk_total.value from disk_used, disk_total where host = 'myhostname' and path='/'

The "mydb"."retentionpolicy"."measurement" syntax doesn't work there, either.

Is it a good idea to add aggregation functions for cases like diff(value1, value2) from m1, m2 and divide(value, value) from m1, m2, or should the arithmetic operators be working?

Also, I noticed when experimenting that it's also not possible to divide one derivative by another. For example, if I have two counters, bytes transferred and api calls made - both of which are constantly going up - how would you calculate the mean bytes per api call?

Start with the linked data exploration documentation and reproduce the cross-measurement percentage and derivative queries described in the issue. The issue names no source files or tests; completion would require a decided query-language behavior for arithmetic across measurements and corresponding validation.