Checkering in an unto itself is not easy, but taking your time to do an accurate layout and have very sharp chisels will help a lot! I makes sure all of my lines are scored 1st using a good straight edge with a cork bottom so it doesn't slide around. I then use a 1/2" chisel and do my short checks 1st, then I switch to a 1? chisel and do my long runs, taking great care to track closely to my last checker to be sure I carry a nice clean and straight line. Truth be known, some wood cuts so much better and some wood, is down right nasty (bloodwood), a lot depends on the grain and the natural tendencies of the wood species. Obviously as you have found, practice and experience makes a gig difference. As far as the angle of your cuts, well, unless you are using a mill, it all comes down to training/coordinating your eyes and hands to achieve a consistent angle/feel for the depth you want for tuning and for what works/looks best.