Oxford1G
This might be too late for your project, but here goes:
1. If the pilots are filling in a few loadsheets a day, then 100 pilots * a few * 365 = many load sheets per year. So we can use 'large population' statistics.
2. For an accuracy level of plus or minus 5 percentage points, the number of loadsheets needed to be checked is 384. Survey the 384 load sheets, and (assuming you get the figure of 20% errors), you can then say that you are 95% sure that the true percentage of inaccurate load sheets is between 15% and 25%.
3. For an accuracy level of plus or minus 3 percentage points, the number needed is 1067. You can then say that you are 95% sure that the true percentage of inaccurate load sheets is between 17% and 23%. 2 percentage points is 2401 samples.
4. Go to
http://www.geocities.com/profemery/samplesize.htm for further calculations.
It would only be accurate to transfer the results if the other aircraft had very similar loadsheets and computations and pilots. For example, I guess that a Navajo Chieftain and a Cessna 441 would be fairly similar, but a Navajo and a 737NG would be rather different.