How many exemptions to claim

Hello, my first question is:

We file jointly taxes, children do their own income, no longer dependents. I used to claim 2 but last year we had to pay a good amount back to IRS so, should we claim zero now or that number could be different if we use the calculator?

Second question: is there a new form now to make changes? If so, would that form make it easier for me to determine what I should claim or the calculator is still the best bet. Thank you!