Basic use of query operation

Hi, thanks for this module. I intend to use it for count of vacations days.

I can't make "query operation" work (using admin/settings/Views Calc)

Here is the example :
Node as field 1 and field 2 (both decimal) how can I get a views calc calculated field = field1-field2 ?

is this syntax correct ? : %node_data_field_1.field_1-%node_data_field_2.field_2 (in query operation textarea)

with some tests I get no result in the view display (blank)
with other test I get unknow column ...in query

Does the fonctionnality work in 6.x-1.3 ?

thanks for help.