Help on UserForm control

Hello all !
I want to create a userForm to make my data entry easier on a sheet with formulated data. If you may try the sample xls I attached, I really want my form can populated the cell C2 but since there's already data in cell F2, the code input the data in cell C11. Is there some other code I can use so the form can specify populated on last empty cell ? Sorry my explanation. But i think you can get the idea when you try the code.

Thank you in advance !