Find last row in excel macro worksheet

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C#

MS-Excel

DataGridView

export

I have windows form application. It has a datagridview in it that tracks time and activity.
I have to save every new row into and excel file that is formatted and runs a macro.
I have tried to locate the last row of the sheet where i have to "copy" my new rows string.
But when i run the find in the range it throws row 10015.
Could it be because of the macro prepopulates certain rows? If so how can i find the last used row between those?
Any help is greatly appreciated!