total.. is based on the agregate function of field
i'll try to explain :
Your example says ;
use field total..20 / total..19 * 100 only when total..19 is not zero.
The total.. fields are maintained in order of using the aggregate function TOTAL in a report field.
The total.. fields must not be in order of reportfields.
Report with 3 after.. layouts.
In 1st layout you use the aggregate function on 1st and 5th field, so totall..1 and total..2 ist used. After this you set the aggregate on 3rd field in this layout, total..3 is used.