As an alternative, you could do as outlined in the image below:
In the survey form of your xlsform:
In the choices tab of your xlsform:

Data entry screen as seen in Enketo:
Reference xlsform:
Calculation.xlsx (11.6 KB)
As an alternative, you could do as outlined in the image below:
In the survey form of your xlsform:
In the choices tab of your xlsform:

Data entry screen as seen in Enketo:
Reference xlsform:
Calculation.xlsx (11.6 KB)