ESPP gain calculation

I have purchased shares through my US employers ESPP program where the gain has been added to my ordinary income at each event and taxed. I'm planning to sell them all this year but wanted to clarify the gain calculation for the self assessment. Example: the market value was 50,000 and I paid 45,000 with the 5,000 taxed as ordinary income. I'm selling them for 60,000. Is the gain calculated as 1) selling price - market value (50,000) OR 2) selling price - price paid (45,000)?

Hi,
If you dispose of the the shares at the time they vest, there is no capital gains tax.  
If you hold on to them before disposing of them, then there could be a capital gains tax liability, if disposed of for more than the vesting value.
Thank you.

I recall the price paid is the discounted price offered by the employer so the employer will add the difference between the Purchase FMV - Purchase Price as benefit-in-kind on payslip, Purchase FMV or Acquisition FMV may be the right price with selling price to establish the CGT