I was in a similar situation to Audrey - I have self-employed income and it varies according to my business schedule. In 2008, all of my income was employee income and a *lot* more than what I made self-employed. I ended up paying taxes to the IRA, but the overall tax burden was less than half that of the previous year, so based on my reading of the rules, I'm OK.
I use the AI method for my tax calculations - mine is a bit easier as I have Foregin earned income, so only worry about the SSN and Medicare until a certain threshold of earning is reach and to be honest, I'm endeavoring to stay under that.
The only other variable is the increase in investment income needing to be accounted for in the estimated taxes - as that grows, then I will need a better estimating method .....
Deserat aka Bridget
“We sleep soundly in our beds because rough men stand ready in the night to visit violence on those who would do us harm.” - George Orwell/Winston Churchill