First, calculate your current eDPI:
eDPI = Old DPI * Old Sensitivity = 800 * 0.5 = 400
Now, use the formula to find the New Sensitivity:
New Sensitivity = (Old DPI * Old Sensitivity) / New DPI
New Sensitivity = (800 * 0.5) / 1600
New Sensitivity = 400 / 1600
New Sensitivity = 0.25
So, your new in-game sensitivity should be 0.25 to maintain the same feel.