If average cost per conversion contributes to 50% of your business targets, then you should not allocate more than 50% of your marketing budget to digital marketing. It means that the remaining 50% of your budget should be allocated to traditional marketing from where you should generate at least 50% of business revenues. Combination of bottoms up approach and top down approach is recommended for effective digital marketing plan. For more details, you can read this article to know the foolproof strategies for digital marketing