configuring a monitor for metrics with multiple queries

I'm relatively new to sumologic metrics, and I am trying to add a monitor for a situation where i have multiple metrics queries, but i only want to alert on a single one of the queries.  the situation is essentially like:

#A: <query a metric for a total>

#B: <query a metric for an error count>

#C: (#B / #A) * 100

basically, #C generates a percentage metric, which i would like to monitor.  however, as far as i can see, when i configure a monitor, it will alert for any value in any query going over the threshold.  in this case #A and #B have lots of values over the threshold, and i don't want to alert on those.  i only want to alert when values in #C go over the threshold.

is there some way to either put all queries into a single query (i don't think so), or configure a monitor to only alert on one of the queries?  thanks for any help.