Dashboard- show when an incident has reached a percentage of SLA

Summary:

We are looking at changing a dashboard to show when a response has not been given at a certain percentage of the SLA

Content (required):

The Dashboard will highlight in colour if the response has not been sent in 10% (amber) or 20% of the total SLA time. We have been doing this with an exception (listed below).

This would be easy if we could use rel_date_diff in an exception: rel_date_diff(incidents.initial_soln,incidents.created, incidents.rr_id)

But we can't see a way to do this as it can't be used in exceptions or functions.