[Finnucane]

When I was freelancing I'd put a third of every client payment in a separate savings account. Wasn't really a sophisticated calculation, just making sure the money was set aside. As pointed out below, the need to make payments out of that account limits you to liquid cash accounts. Putting that money into a risky account to get a little extra interest seems like asking for trouble in the long term.

[Gibbon1]

I also withheld a third and it was good enough. I would deposit checks in a business account and then transfer 2/3 of the amount to savings. I'd also pay for anything business related from the business checking account/card.