There is no exact way to figure the appropriate level of umbrella coverage you need. Your first impulse might be to protect your assets, but this figure is practically irrelevant. A judgment against you could easily wipe out your assets and put your future earnings in jeopardy. Instead, consider factors such as how often you have visitors at your business, how much you and/or your employees drive for business, and whether your business could give the impression that you have deep pockets.
Coverage limits vary, but a typical policy will provide $1 million to $10 million worth of liability coverage.