UK Supplier Rebate modelling (Excel)

[Synapse1975]

Anyone have recommendation for building a supplier Rebate model which is based on proportion of business (volume) tiers. Looking to incentivise current customer who we have 40% share with, and incentivise to transfer more business from other competitor (who holds remaining 60%).
Is there a non-linear approach to reward immediate volume growth and taper off at back end. Thinking need to incorporate overall contribution in setting bands (or assessing impact), but not sure how to approach developing such an incentivising model. Any help appreciated