I receive an error message on my parent cell formula because I am using sub items that have mirrored values. Can the use of multiple mirrored value formulas be added to a future update?
In the meantime, while monday.com are working on the constantly evolving and improving features, you might find column magic copy to be useful, depending on your set up.
You can take the value of a mirror column, paste it into a text or number column to then be used in place of the mirror column for automations as described.
Feel free to reach out if you wanted to discuss together 
peta@upstreamtech.io
I’d like to add if you’re working with numbers, consider Mirror Magic: Numbers. Its from the same devs as Column Magic (of which I am one!) but it can do a few things with mirrors of numbers that you can’t in column magic. It also can support multiple mirrored columns (up to the limit monday allows at least) directly - and you can select if you want the sum, average, min, max, among other operations on all the mirrored items.