The Lindsay tool is good for where you do not have a direct measurement - you are basically asking people's opinions. In itself, the tool can represent an overall single number from multiple questions if you lump all of the questions into one set of data, and separate it by month. Use the technique, treating each month's cut as a separate "question" and you have a trendable single number.
Whereever you can get an actual measurement, that is usuall best. Customer complaints, repeat business, referrals can be an element. You can meld these together as a single indexed value by adding together the indicators with weighting factors, but I prefer a "balanced scorecard" approach of overviewing the individual metrics. See at the same website the presentation on color coded SPC dashboards.