Use this calculator. Put "90" in the "Net Actual Aquarium Water Volume" field. Put "420" in the "Current Calcium" field. Select "ESV B-Ionic Original Ca Buff Pt 2" in the "Product" field. Adjust the number in the "Desired Calcium" field and hit the "Calculate" button until the "Product Required" field shows 25 mL. Your answer will be "Desired Calcium" minus "Current Calcium".
Thanks, I did that prior to asking the question and it gave me 4.5 ppm. I was just verifying that I was inputting the numbers in the correct spots. Thanks!