I was reading an article written by a ‘3D Economist’, [whatever that is] which purported to negotiate a renewal of a commercial lease as ‘per the market’.

Under contract law, of which the sale of land and other property matters fall, there is a requirement for certainty [of price] within the contract. If there is no price stipulated, then, an objective mechanism for establishing that certainty of price is required; Fletcher Challenge Energy Ltd v ECNZ [2002] 2 NZLR 433, (CA) at [53] and [63].

So a ‘market rate’, as long as it is expressly provided for is sufficient, but, it seems a risky way to negotiate a commercial lease renewal, as, the market rate is always ascertainable and can be used, but the market rate may be a wholly inappropriate way to value a renewal clause in your specific contract.