Which share price - after or before discount - should I use to calculate my CGT?

See section 8 with examples in the following Revenue reference: https://www.revenue.ie/en/additional-incomes/documents/it72.pdf

Excerpt:
An individual who acquires any shares by the exercise of a share option is chargeable to CGT on any chargeable gain realised on the subsequent disposal of those shares. The tax is due on the difference between: (a) the sale price of the shares; and (b) the acquisition cost of the shares.

The acquisition cost of the shares for CGT purposes is calculated as follows: Where the shares are issued on the exercise of the option: Regardless of when the options are exercised the cost of acquisition is the sum of the following:
The cost (if any) of the option,
The price paid for shares on exercise of the option, and
The amount charged to income tax on the exercise of the option.