Impurities calculation approach in Eu. Ph

Hi everyone. I’m new and I would like to ask you a question. I should decide which method is better for calculating methylparaben related impurities. The method speaks of “comparison” of areas but having the diluted sample as a standard I would use an RF. Impurity A has a factor of 1.4. Questions:
Is it right to use external calibration or not?
Why sample solution is diluted?
Where do I found documents that regulated calculation of impurities?

thank you!