In the filtering process. The amount of oil you have is the key. People assume that a 4gm kit comes with 40ml of oil, so they go "4,000mg into 40ml = 100mg/ml".
However, a 4gm kit comes with 50-56ml oil. Therefore a 4gm kit with a 3% loss factor = 3,880mg of Fina. 3,880mg divided into 50ml of oil = 77.6mg/ml.
If you want a higher concentration use less oil. That will increase the relative concentration of BA, which could make the shots more painful.
I wanted as close to 75mg/ml as I could get so I used all the oil. I inject two ml's every other day.