Formula question

You may also need to deal with rounding of the calculation since most extensive tax calculations round the results of a calculation to 2 decimal places. While Acrobat forms may display the rounded calculation amounts the actual value of the computed fields will extend to many more decimal places and these values beyond the 2 decimal places can change the result of calculations being performed after the tax calculation. The applying of rounding a calculated amount to actual field value cannot be done in the Simplified Field Notation.

It's not a sum, it's a product...

It appears you want to calculate the sum of the field Net and the result of the calculation of the fields Net times Tax.

This can be done using the Simplified Field Notation

Net + (Net * Tax)

or

Net * (1 + Tax)