Assuming you did sell after 10 years, this is what your spreadsheet will look like for the first year of monthly purchases. If you didn't sell you would have a column at Year 16, 24, etc.
In your annual tax return you will sum up the tax in the 12 rows. You will have one of these sheets for each calendar year of purchases.
| A | B | C | D = A * (C - B) | E = D * 41% | F | G = A * (F - B) | H = G * 41% | I = H - E |
Purchase date | Nb of shares | Price @ Year 0 | Price @ Year 8 | Gain | Tax, where >0 | Price @ Year 10 | Gain | Tax, where >0 | Net tax to pay |
01/01/2024 | | | | | | | | | |
01/02/2024 | | | | | | | | | |
01/03/2024 | | | | | | | | | |
01/04/2024 | | | | | | | | | |
01/05/2024 | | | | | | | | | |
01/06/2024 | | | | | | | | | |
01/07/2024 | | | | | | | | | |
01/08/2024 | | | | | | | | | |
01/09/2024 | | | | | | | | | |
01/10/2024 | | | | | | | | | |
01/11/2024 | | | | | | | | | |
01/12/2024 | | | | | | | | | |