The problem with doing only obly one run for each film is that you can't determine what error limits there are. Statistically they can be very high. In an analysis there are two factors; accuracy and reproducibility. You can have a set of values that have a very high degree of reproducibility but are yet far from the real value. Conversely you can have a set of values with little evidence of reproducibility whose average is very close to the real value. Without doing a statistical analysis any data is suspect. Your analysis is better when you include Xtol since it can be said to be a standard and validates your method. At a very bare minimum you need two sets of data. A spreadsheet can take away the drudgery of the mathematics. I personally would stick to tier one films since their quality control removes another variable.