Blonde to Ombre

I have bleached my hair to a super light ash blonde for the past year or so. My hair grows really quickly so i need to do my roots every 4 weeks. This is leading to a bit of breakage around the hairline and obviously is quite a lot of upkeep.

I have decided the best thing to do is a sort of root stretch/smudge ombre as I want to keep the blonde as much as i can. I would say my natural colour is a 7.
I have bought a couple of colours that seem to match my root - they are semi perm!

I am going to attempt to stretch my roots so that I can go bleach-free for as long as possible to give my hair a rest. Any tips on the best way to do this so I don't end up with a BLOCK of brown and then my blonde? I figured I will comb the brown through slightly so there isn't such an obvious line. Also i'll leave some blonde strands all the way through the front. Other than that any tips would be great!