Hi,
This is what I used in a recent form to calculate average:
Select 'type = calculate', set some variable-name in 'name' column, and in calculation column, put following:
round((${learners_benchmarking} + ${parents_involvement} + ${parents_participation_in_CEC} + ${local_community_involvement} + ${school_resources_sharing} + ${CEC_interest}) div 6)
The values of variables are coming as numeric input from the questions above this row.
Remove the ROUND function if you don't want rounding off.
Hope it helps.