If a retailer’s rental payment for the premises described in a contract entered into under s. 565.10 is contractually computed, in whole or in part, on the basis of a percentage of retail sales and the computation of retail sales is not expressly defined, under the rental contract, to include sales of lottery tickets or lottery shares in the lottery under this chapter, any compensation received by the retailer under s. 565.10 (14) shall be considered the amount of the retail sales from the lottery tickets or lottery shares for the purposes of computing the rental payment.