If shutter offset is already -90 you shouldn't have an issue entering a negative. That's the value you're talking about, right? Or is this a comparison of your work workstation vs your home one? where the work one is set to -90 but the home one can't go negative?

Either way, your issue is almost certainly the same as the OP: adjusting the shutter offset to match the footage. Play around till it looks right. You should be able to tell by individual low quality renders. Just IPR an object with a medium amount of motion and change the setting till the blur locks into place. I think.