Applying bleach to ends for ombre/dip dye

Hey y'all  *waves*

Long time, no post.

I've been natural (aka mousy) in the summer (achieved with a semi permanent box over faded funky, pre bleached locks), auburn in the autumn (L'Oreal box dye applied 05/09/14) and then a bit sassier red in winter (yep, another box dye, this time Garnier Nutrisse ultra colour 10/11/14)

So I have been using red laden boxes but I have been leaving it 8 weeks between dyes.  My last bleach was June.

I have been growing out a pixie crop and now have a kind of graduated bob with the back layers showing about an inch of under hair at the nape and the longest side bits a cm or two past my ears and a long side fringe.  I'm SO bored with the funklessness and I was thinking I could lighten up my healthy and slightly faded ends in order to deposit good colours then pull some chocolate brown through my mousy roots and the top few inches of red to create a dip dye/ombre kinda thing.

If anyone has any advice on placement and separation then I would appreciate it.  I think I have previously gathered hair in a bobble to remove the straight line of bleach and applied it with a haphazard hand when doing ends and gradients but my hair is so much shorter now.

I have only whole head experience at this hair length but I am trying to minimise damage so lifting the whole lot before depositing the dark again seems risky.  (I once did a full head of that ghastly bleach and colour stuff then pulled brown through to create reverse pink highlights but that stuff is lethal).

Sorry I have rambled, just trying to give a full picture to gain accurate advice if anyone has any?  *anyone*