Formula for diminishing lease value

Tax Planning and Financial

I own a leasehold property which I have owned for 31 years and which now has 68 years left on a reversionary lease. Given my age and the borrowings It is likely I will wish to take the hit on the CGT in the next five years and sell the lease. Is there formula to work out the level of reduction in value of the lease as it moves from 68 to 63 years left to run? I understand there are a few variables but the BTL mortgage is due to renew and I need to take a two or a five year view. It would be great if I could balance the income against the reduction in value of the lease before making a decision so a formula may be good