Contact
Corrections to any figure on the site are the most useful mail this address gets.
How to reach a person
Email is the way. contact@concreteslabcalc.com is the only address the site has.
Please include the page you were on and the dimensions you entered. Almost every report that turns out to be a real defect is a rounding rule or a constant, and both of those are visible from the numbers you saw.
What this address is not for
It cannot quote your job, source your concrete or tell you what thickness your local code requires. Those need a supplier and a building department, in that order.