I think it`s great that you want to contribute to charity from your business results. Some things to consider:
1. A percentage of the annual sales may not be the optimum way to decide how much. Consider contributing a portion of the operating profit or profit before tax.
2. If you intend to get start up capital from "investors", they should agree with your intention and you should disclose it to them.
3. If the capital will come from borrowing - you need to be sure the revenue will service the debt. See #1 above
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