Polarization and the percentage of up ($\uparrow$) spins:

$$\mathrm{Polarization} = \frac{I_\mathrm{up} - I_\mathrm{down}}{I_\mathrm{up} + I_\mathrm{down}} = 2 P_\mathrm{up} - 1 $$

$$P_\mathrm{up} = \frac{I_\mathrm{up}}{I_\mathrm{up}+I_\mathrm{down}}$$