I'm divorced w 2 kids. I have them 80% of time but ex and I each claim 1 on taxes. Do I need to calculate the total I spent on preschool for the kid I do NOT claim?

[xmasbaby0]

Only the custodial parent can claim the childcare credit.  It sounds like that is you.  Why are you not using a form 8332?  Is there a court order that compels you to only claim one child?

Are you the custodial parent?  Do you have an agreement with the other parent to allow the other parent to claim them--due to divorce or that you live apart and share custody?  Did one of you sign a Form 8332?

If there is a signed 8332 then the custodial parent retains the right to file as Head of Household, get earned income credit and the childcare credit + education credits if the child is a full-time college student.  The non-custodial parent gets the child tax credit for children under the age of 17.

As far as the IRS is concerned, the custodial parent is the one with whom the child spent the most nights during the tax year--at least 183 nights.

[macuser_22]

Only the parent that lived with the child more than half the year (custodial parent) can claim the child care credit.

Child care is "work related". It can only be claimed if the care was necessary so that you (and your spouse if married) could work. Only if the child lived with you more than half the year *and* the care was required so that you could work is it allowed. If you did not live with the child or work at the time that the care was provided then it was not necessary so that you could work and therefore is not allowed.

See IRS Pub 503.
https://www.irs.gov/publications/p503#en_US_2018_publink100048433