To close this subject, it was recommended to me I double check the use of the field name: it is very important in mathematical calculations to use the 'Permanent Name' of the field (Edit Field > Adanced > Permanent Name), then calculations work perfect!
- permanent names will almost always differ at least slightly,
i.e. 'Name' may = 'name',
- if the name has two words, it will almost always differ
i.e. 'First Name' may = 'firstName',
- if not English (i.e. latin alphabet) Tiki will always change the permanent name of the field name,
i.e. 'f_356' (which resolves all language issues!)