I generally monitor three if I open Task Manger for that purpose, though I'm generally unconcerned with the avast memory usage as it is comparatively light for avastSvc.exe and avastUI.exe the two avast entries.
1. working set
2. private working set
3. Commit Size
Personally I see no point in monitoring Peak anything as it is a necessary evil when avast scans memory usage will increase. So I don't monitor peak working set.