Calculating Signal Approximation Error (Theoretical signal - Real hardware signal)?
There are lots of different error statistics you depending on what you want to know. If what you care about is the maximum error then what you suggest is fine.

Probably more common is to use the RMS error i.e. $ \sqrt{\frac{1}{N}\sum(Y-Y_{real})^2} $. This is related to the variance of the system. I'm not sure if this is exactly what you want for your case as I suspect there will be strong spatial correlation across the filter. This is something you should look at.

Another issue is that the error is dependent on the input signal. You may wish to look at the frequency response of both systems to try and remove this issue.