multi DB

After reading the first half of https://premium.wpmudev.org/project/multi-db/installation/ I'm confused.

I have cpanel with host gator and each database has a prefix which is the cpanel username
i.e.

cpanelusername_

after the _ i name my DB

my wpmu database is named cpanelusername_WPMU so is that what i put in to the tool at https://premium.wpmudev.org/db-tools/db_sql.php or do i just put WPMU?

thanks much

side note - i installed 5 plugins should i delete them all and then install this one first ?