Assuming you haven't changed any of the multipliers in the ini file, you would enter your sensitivity into the conversion calculator as you normally would, then also set the "multiplier" which by default is .02
It should also be noted that if you want to match CS:GO's FoV in Siege, the setting would be about 74(73.74) vertical degrees (which translates to 90 horizontal on 4:3)
EDIT: You can also select "Hipfire (Simple)" from the drop-down menu and it will enter the default multiplier for you.