Everyone's backpay is going to be different based on what their monthly SSDI or SSI rate would be multiplied by the number of months that they are owed. Does $541 a month for 48 months sound about right to you ? That monthly amount could look low because I divided by 48months, but you'd want to account for the SSA not covering for the first few months and/or if you haven't been out of work quite that long.

I believe SSI is usually around $650, but your's could be less because you've been getting partial disability. If you call the SSA office, they should be able to explain your numbers for you.