Mathieu, depending on your system you should first try to leave MIR Pro's own processing latency at 0. Set the number of threads assigned in VE Pro's preferences to the highest possible value. Now try the different ASIO latency settings or the RME (512 is most likely a good starting value). Now try how far you get with these settings. If you encounter crackles due to CPU overloads, switch MIR Pro's latency to at least twice the size of the ASIO settings (1024). If this works, you can try to reduce the number of threads again (which might be necessary when you run other applictions on the same machine).
If you still have to change the ASIO settings to some higher value, remember to increase the MIR latency settings, too.
This way you should find the optimum settings for a given arrangement size. Just remember that they are no fixed values, they depend on a multitude of variabels.
HTH,