Mary, hate to be the bearer of bad news but I think you need to financially prepare yourself for an overpayment notice from SSA. That, unfortunately, is not how the law works.
You have nine trial working months- PERIOD. A trial working month is ANY month (for 2011) that you earn over $720 a month. EACH MONTH that you earn over $720 counts as 1 trial working month. Once you have accumulated 9 trial working months in a 60 month period, you have exhausted all of your trial work months.
Then, if you earn above $720 but under the "substantial" amount of $1000 per month in ANY of the 36 months following your LAST "trial working month", then you will still get your benefit. So if you've exhausted all 9 trial working months, but then end up in the hospital and only earn $650 for one example, or $850 for another example, you still get your FULL benefit for that particular month.
There is no SSDI income reduction based on gross. This happens with SSA RETIREMENT, but NOT disability. The only reductions given from gross income are for out of pocket medical expenses or self employment expenses to reduce yoru gross to keep it under the "substantial" amount or the "trial working" amount.
You guys really need to familiarize yourself with the law:
http://ssa-custhelp.ssa.gov/app/answers/detail/a_id/317/kw/trial%20working