1. Add a bubble level of some sort to your set up to eliminate canting errors.
2. Eliminate parallax errors by carefully checking your focus and marking the scope's focus ring. Best done with the scope off the gun IMO.
3. Consider either adjustable mounts, an inclined base or doing the epoxy putty scope ring mod. This will ensure you have sufficient vertical adjustment available from the scope to allow you to dial the range. Having your POI along the horizontal stadia makes aiming off for windage more repeatable.
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Barrel cleaning: Whilst many users never clean their bores every rifle I've done has shot tighter groups afterwards and with many fewer flyers. I just use some regular VFG felts with a little pellet lube/Lupus grease, then a couple of dry ones on a VFG cleaning rod for a normal clean.
If that doesn't do the job then I use the intensive felts with VFG Bore Paste before moving on to the regular clean.
The worst "new barrel" I've seen was on an HW100. Here's what the felts looked like coming out of that one , shot properly post cleaning.
If using a fishing line pull through then a bore guide is a good idea, dacron etc will happily cut steel and ruin your crown in no time.
Give the SSC a go when you are up and running. HTH