Add 0.5, 1.0, 3.0, 5.0 and 10.0 units of enzyme to each test points. Incubate at 37 C for 2 hours. Analyze the results on agarose gels. Almost every piece of our YAC DNA are different from each other. Some sites are easier to cut than others.
Impurity varies from sample to sample.
I have used some enzymes were bought 8 years ago. I do not recommend used them directly but test them first. That is to say when you trying a partial digestion reaction, you should start with 0.1 U per reaction and end with 0.0001 U per reaction. U to 0.001 U per reaction.