Without a chosen mechanism it remains open who bears the risk up to completion.
Completion accounts and the locked box allocate the economic risk between signing and completion differently. With the locked box the buyer bears the development from the effective date, but knows the price early. With completion accounts the price only becomes final after completion, but the buyer pays the actual position. Settle this fundamental question before you negotiate figures.
An initial orientation on price structure is provided by our post on the earn-out clause.