There are a number of online tools and calculators which can assist you with estimating the cost of your lease extension and we would refer you to the calculator on the LEASE website https://www.lease-advice.org/calculator/ please bear in mind this provide you with an estimate only.
To obtain an accurate indication you will need to instruct a specialist valuer with expertise in leasehold valuation, and this is usually the starting point for a statutory lease extension.
If you are negotiating a premium directly with your landlord then there is no rule of thumb, but the premium will usually be by reference to the cost of lease extensions carried out on other properties within the same development, or other similar properties with same existing lease terms.
We would in all cases suggest that you speak to a leasehold valuer to ensure that any premium put forward by your landlord directly is at least as ‘good a deal’ as that which you would likely obtain following the statutory procedure.