Hi, the calculation of the rent payable in a commercial lease is generally a business decision and the subject of commercial negotiations. However, when negotiating the rent, the following factors should be taken into account:

1. Whether you wish to lock in an exclusive fixed space (giving you certainty, but reducing flexibility)

2. Whether you wish to rent space on a variable use basis (which gives you flexibility, but given the space is used by other users, there may be no scope to increase rental space)

3. Whether you have control over where the goods are stored (can space be utilised more efficiently?) - it is not in your interest for the landlord to be able to distribute (say) crates in a single layer and then claim you have used half the warehouse floor area.

4. What common areas are shared (toilets, pantries, corridors)

5. What areas are exclusive (Showroom)

On the basis that your goods would still be conveniently accessible, you may wish to consider negotiating rent on a fixed floor space basis, and not the "overall floor space" that may be taken up from time to time, as this is how many consignment and online businesses function. You may wish to attach a floor plan to the sub-lease agreement to clearly demarcate the area which you have exclusivity.