Welcome to the forum paulgj,
To answer your question: there is no best setting really as it all depends on what you want. If you want to have a quiet drive and don't care much about speed then you can lower the setting. If however you want performance and don't care about noise then use a higher setting. For DAE a high(er) read speed doesn't make any difference. When the drive does encounter an error it will slow down the read speed by itself. If you do DAE with PlexTools you can set-up the error handling yourself as well (allow speed slow-downs, the number of read retries, etc.). Hope this answers your question!
Maybe helpful: PlexTools DAE Guide by westgroveG