Yeah, I know what you mean by not knowing the exact outline - we had to do max 1st molar class II's without being actually shown the outline. Your rule of thumb is always to just follow the grooves, and only open the contacts on the box as much as you need to - any more will take out needed supporting tooth structure, and less won't open the contacts.
Just take a sharp pencil and trace in where you think the walls need to go - if you look at the tooth from above the outline should start to be clear. Be careful though, you may need to drop the box not quite as deep on the distal because the curve of Spee. Man, I just love that phrase... good ol' curve of Spee.