A good baseline would be one based upon real data you have collected.
The first step to doing this is recording all estimates. The second step is recording the actual results. Be honest, don't be tempted to 'self-adjust' the actual. Gather enough of this information and you have some data to statistical base how good your estimates are. Note, this can/will vary wildly based upon who did the estimation and who the work. Only by doing this can you be reasonably expected to give a 'margin of error' that is anything other pure rubbish.
It doesn't stop there either; analyzing what causes estimates to be off can help improve the accuracy of your future estimates. Note: They still remain estimates, and as such, are only estimates.
Estimation also doesn't finish after the first estimate; this is something that can be adjusted as the project progresses as more knowledge is gained, this reducing the possible margin of error as you go along. The more open you are with communication, the earlier surprises are discussed - leading to people been less surprised and allow more time to make adjustments in either the project or customers expectations.
Second, perhaps a better way of handling error margin is 'confidence internals' rather than merely % margins of error. You than based you estimated delivery date on a confidence interval, rather than a singular date.
PERT is one example of a framework to handle estimation based upon statistical reasoning. For example:
"Based upon what I know now, we have a 90% confidence level we can deliver in 8 months. 95% confidence in 10 months, 99% confidence in 2 years, etc"
Note here: The more confident they want you, the larger the estimate will be. Depending on upon the complexity (aka, how accurate you could possibly be), it could be a small difference between 80% and 90%, or it could be huge!
Lastly - Good luck ;) If you ever solve 'maximum error margin' in software estimation whereby you can specify something like 'all our estimates will be +/- 10%', be sure to commission a box-office movie for the rest of us in the software industry. I'm thinking something like a cross between Office Space and The Matrix :D