Well, which Verbatim DVD+R DL are you using? MKM 001 (2.4x) or MKM 003 (8x)? And, which burner are you using?
I'm not sure there is a general guide. For example, in my experience MKM 001 burns best at 4x in my Optiarc AD-7260S and all my other burners. 2.4x actually gives drastically inferior results to 4x in all the drives. On the other hand, MKM 003 burns best in my Optiarc AD-7260S & NEC ND-3540A at 4x. However, MKM 003 burns best in my Samsung SH-S203B at 8x.