Hi@Kmayes,
Based on the issue that you mentioned, do you want to check the Checkbox based on the DataCardValue30 or update the DataCardValue30 based on the Checkbox value?
I find that your description is inconsistent, could you please confirm the requirements?

1). If you want to check the Checkbox based on the DataCardValue30, please set the Default of the Checkbox:
If(DataCardValue5.Text="Yes",true,false)

2). If you want to update the DataCardValue30 based on the Checkbox value, please set the Default of the DataCardValue30 as below:
If(Checkbox2.Value,"Yes","No")

Regards,
Qi