1. each row must total 45
2. each column must total 45
3. each 3 x 3 must total 45
4. I created an spreadsheet to automatically track my entries and each total, so I don't have to remember what remaining numbers I have to deal with in either a column or a row and how far away from total I am in any given row / col / 3 x3
The rest is blood, sweat and tears...