OUF_Assassin is a project mainly written in Lua, it's free.
An oUF layout to make you not bad at remembering to Backstab at 35%
A Display to remind you to switch to backstab as 35% for mut rogues. Height and Width are configurable, other than that, there are no options. If you want to move it, get oUF_moveableframes by haste on wowinterface.com