Letting relief reduces your taxable gain. It's in HS283. The calculations are much simpler than people think.
Gain: £55,050 (£190,000 - £134,950)
Total period of ownership to 5 April 2020: 78 months
Period of occupation by you: 35 months
Period of letting: 43 months
Exempt gain: £37,406 (£55,050 / 78 x 53 (35 months + last 18 months of ownership)
Letting period gain: £17,644 (£55,050 / 25 months (43 months - last 18 months of ownership)
Gross Non-Exempt Gain: £17,644
Letting Relief: £17,644 (lesser of £40,000, £37,406 and £17,644)
Net Gain before annual CGT exemption: £0
Provided you sell by 5 April 2020 you will have no CGT to pay.
If you sell after 5 April 2020, you will lose the letting relief of £17,644 and £6,352 of the last 18 months relief, ie you will have a taxable gain of £23,996. Deduct the annual CGT exemption of £12,000 and you would pay CGT at 18% on £11,996 (£2,159.28). The gain is added to your income to determine whether you pay CGT at 18% or 28% or a combination of the two rates.
If we have a change of government in December 2019, CGT will probably change quite significantly. I suspect the rate will go back to what it was in 2008, ie 40%.