@Brenda_Snider You will need to get all of your data on the same line in order to do the math you want.
Based on your existing setup, probably the easiest & quickest way to solve this would be to figure out a way to link the check out record to the check in record using a linked record field. Once you do that, all you need to do is create some lookup fields to bring in the data that you need onto the same line.
Since the ID’s are both the same for both records, and the check out record will presumably always be created after the check in record, I would simply create an automation that is triggered based on the “check out” checkmark.
Once the check out checkmark is checked, the automation should search for that same ID, but search for the check in record that matches that ID.
When it finds the matching record of the check in record, take the record ID of the check in record and put it into the linked record field of the check out record.
Then you can create lookup fields to bring in the check in fields onto the check out record.
So all of your math will be done on the check out record’s line.
p.s. Note that all of this will only work if you have one check in record and one check out record per ID. If you’re going to have multiple check ins and multiple check outs based on the same ID, then a significantly more complex setup would need to be figured out.