in the attached,
- Adjust the green cells' values in row 4; this is the daily increase in weight (can be negative)
- Adjust how long that increase should apply for in the green cells in row 2
- Hide row 3
- Change starting weight in cell C7
- The formula in cell D7 is the one that can be copied across as far as you need.
- Row 13 is only there to demonstrate how the weight increments have been applied, can be deleted.
There's probably a slicker way.