And it's not quite that simple, is it? The part of compensation that is not deferred is taxable income. The FICA amount is subject to federal income tax at a rate of 10%, as is the amount paid as income tax, so income tax = 0.1 x (FICA + income tax), or income tax = FICA / 9.0.
For compensation of say $1000, FICA is $76.50, and federal income tax is $8.50. So, there's $915.00 left to defer. If there are other deductions, such as, for insurance, state income tax, etc., it's more complicated, and there's less left to defer.
Bottom line is that Bill Presson's general rule is a good suggestion, and I'd lean toward 80% or 85%, because 90% may be cutting it very close.