Need help running the calculate event depending on a checkbox value

Hi all,

I'm having trouble incorporating the calculate event to execute depending on whether a checkbox is checked or unchecked. Seems like any way I go, they will not play together. This might be easy for a JS expert but its not working for me. So below is a link to the form and I have stripped it of anything I've tried that didnt work so you can have a clean slate. I have two tables on this form that are essentially the same thing so if we can get the top one working I can get the second one working on my own.

Here is how it should work. The repeating row in the first table (tblWarrants) has a Quantity (quantity) and Unit Cost (unitCost) fields when multiplied together will show that value in either the Maint. Cost (maintCost) or Engr. Cost (engrCost) fields. What determines this is if the checkbox (cbEng) is checked or unchecked. If cbEng is checked then I need the calculation to run on the engrCost field and show the value. When the cbEng checkbox is unchecked then I need cbEng to be null and the maintCost field to calculate its value. Upon opening or by default, I need the cbEng to be unchecked and engrCost field to be empty/null. 

https://drive.google.com/drive/folders/10wPFynb1frPGLHtdwaclgRTUDfyYaHek?usp=sharing

Thank you!