Have fridge running while checking the freon or shut it off?

I`m sorry,but if you don`t know ,this has to mean you haven`t had training, if you don`t have the training and don`t have a EPA card, you are breaking the law by handling or dealing any way with refrigerant. I don`t think you will get a certified ,EPA trained tech to anser your question,, I/we could be fined along with you

Shut it off. There are web sites telling you how to charge a refrigerant system