You will need to get the land properly valued. The freehold land will have a value and so too will your leasehold interest but I do not th8nk that you can estimate the land value as simplistically as a 1/3 of the property value.
As to the split between leaseholder and freeholder, this will ultimately be down to negotiation between the parties.
I assume from your last question that the land is to be sold to a developer, if possible. I could not see subletting being viable if development is proposed. Far too many issues to make it unworkable.