If replacing the existing tweeter, same value as before.
If extending the existing tweeter, you need to pick up where the existing tweeter begins to both drop off and become spikey. This requires a band-pass for the existing tweeter so you remove the distortion at the end of the range. Basically the same approach as a midrange. So you are making a four-way.
Frequency for a high-pass filter is:
f = 1 / (2 x Pi x R x C)
where
R is the nominal impedance of the driver (16
Ω || 16 Ω = 8 Ω) and
C is the known capacitance in Farads (divide by one million to yield uF). Given
f and rearranging for an unknown
C, yields:
C = 1 / (2 x Pi x R x f)