Acrobat Form Fields Not Working

Good morning everyone - i have a pdf form that has 12 fields - 5 are hidden - its a calculator of sorts.

The user puts in their annual earnings, and based off of field3 - field4 is supposed to dive field3 by 12 and generate a number - but it's not working. i've tried this in the in the Simplified Field Notation area:

Annual_Bill_Rev/12months

the 12months is a hidden field with a default value of 12. If i can figure this out, i can probably figure out the other elements as well. so that i may get those other fields working.

i need this by End of Day today so i suuuuuuure hope someone can shed some light on this for me. Pretty please?!

Thank you in advance for any help you can provide.