What's your most valuable tool when calculating payroll?
Honestly, excel spreadsheets/calculators that I have made to mimic the payroll system so that I can double check any paycheck that needs it without manually doing so each time. Is it overkill? Yes, for some. But when you work alone with no real doublecheck (other than the employee at the time they are paid), it has saved me some large mistakes in the past that were corrected before anyone else laid eyes on them and before they became a large issue.
Common sense and diligence. Common sense to not be handling payroll unless I could perform it manually (so I had a rough idea of the expected results, and could catch blatant or repeated errors),. Diligence to not skimp on checking things over before releasing the pay. Diligence not do do less than required by the controlling regulations, and not more than required by the controlling regulations.
In my opinion, one of the important best resources for payroll such as the smart -sheet and ms - excel advance.
The Payroll Source Book by the APA.
Also a good report writer which can run customized proofing reports.