There could be several ways to study a subject well. My views are based upon my preparation base, which got me 79 in costing.
1) The current practice manual is humongous and so suffices the requirement by almost 70-80%. The other reference book that could be referred is Padhuka as it has proper theory and practical explanation to concepts as well as a great number of sums to be solved which covers majority of PM alongwith (so the PM gets reduced by half while solving it alongside)
2) Theory portion is something that fetches around 25-30 marks and takes whole lot lesser time than practicals whilst writing papers and it could be done from PM or last 5-10 papers/RTPs/MTPs. Theory is crucial to achieving a good score.
3) OR/QT though seems tougher but once concept gained becomes pretty much achievable and solveable in exams and hence should be a point of focus while preparations.
4) Never Audit while practising but solve through the sums, a full length solving for few and conceptual based solving for all others.