Manual Entry In Calculations Field

Hello,

It's been a while since I've done a custom script and need help. 

I have a calculations field on a form adding 7 different entries (Contact Duration in Hrs 1, Contact Duration in Hrs 2, . . . Contact Duration in Hrs 7). Sometimes the user has to use two forms because they made over 7 contact. Therefore I want to be able to also add a manual entry option since this form can't automatically add up the values from another document.

I'm looking for something like: if the manual field of 'Total Hours Manual' is blank then display the calculation field 'Total Hours Auto'

- if there is a manual entry then the automatic calculation field should be hidden behind the manual entry

- if there is not a manual entry then the automatic calculation should be visable