Andy when i first started using machines in Germany in the olden day's , i also used to panic about "well how do i get my walls plumb and straight if i'm spraying the whole lot on",, cos i was taught the old picture frame method,... vertical screed in the right hand corner of the room and then working right to left (the correct way for a right hander) horizontals top n bottom and then every three metres another vert screed plumbed up and so on then fill in and rule off , bang on every time,.. but when you spray ,you look for any obvious signs that the wall is out and you try to get an even coverage, then flatten off with the h section (derby) then rule your edges in the same as you would with the picture frame method and then rule through, but you have pretty much got it when you have flattened off with the h section (derby) and are pretty much just tidying up with the rule (feather edge) ... you will not go back to hand applying once you get into the swing off it mate, only for the little bits of jobs and skimming in my opinion