Use IF condition with ISNULL - Microsoft Access

I am trying to calculate a formula but its not workig or i am doing it wrong (obiviously). The scenario is that i have 10 textboxes and a calculation field. I want access to apply a formula on that calculated field (total) according to the data input in those 10 fields.

e.g if the field 9 and 10 is empty then (total) will be divided by 8. Else it should be divided by 9 or ten respectivly. any help in this regard will be appreciated. thanks in advance

what i am expecting if sub9 is null then total/8 if sub10 is null then total/9 else total/10