The back is always the hardest and takes the longest for me.
To help myself I prep my hair by putting conditioner on the relaxed portions of my hair, then using clips to make ponytails. I don't go for the bone straight look, I aim to to get just before straight (which on me is wavy) So I really only process the back for maybe 10 minutes. The top is really crazy thick and takes the most time to process.
The back of my hair definitly a different texture thant he crown, so its not a problem of me doing it last since it processes in no time. I do my nape every other touchup.
IMO, it takes practice and double checking yourself quite a lot.
-A