Converting "MPR121 capacitive keyboard" sensor filter data into capacitance values?
According with the MPR121 datasheet (from NXP), you forgot to include \$V_{dd}\$:

$$ C= \frac{I \times T}{ADC \space counts} \times \frac{1}{V_{dd}} \times 1024 $$

If \$I\$ and \$T\$ are given in \$\mu\$A and \$\mu\$s respectively, then  \$C\$ is in \$p\$F.

EDIT:

ADC counts available on following register:



EDIT 2:

As the valid operating range of the electrode charging source is 0.7 V to (\$V_{dd}\$ - 0.7 V), the min and max ADC counts are given by:

$$ ADC_{min}=\frac{0.7}{V_{dd}} \times 1024 $$
$$ ADC_{max}=\frac{V_{dd}-0.7}{V_{dd}} \times 1024 $$

with a fixed intermediate (mid) value of

$$ ADC_{mid}= \frac{ADC_{max}+ADC_{min}}{2}= 512$$

With \$V_{dd}=\$ 3.3 V (for example on MPR121 Adafruit breakout board), I think that:

$$ \left\{\begin{matrix} ADC_{min} \approx 217.21 \\ ADC_{max} \approx 806.79  \end{matrix}\right. $$

No conflit with the first formula I've presented in this answer.