There are different types of pension and the way of valuing them varies very considerably. A standard personal pension will be made up of cash or investments that are likely to be relatively easy to value at any given point. Public sector and final salary schemes are more complex.
A crucial point to bear in mind is that values change. It's important to use figures that are as up to date as possible and even then there will nearly always be some difference, up or down, between the outcome you're working on in negotiations or court proceedings and what you actually end up with.
There is no substitute for expert advice from your family lawyer who can then bring in independent financial advisers or an actuary if required. Getting it wrong can be very costly and certainly justifies the fees that you'll be paying to ensure you're protected.