What capacitor value should I use for 3.3v from a buck converter IC?
The output capacitor affects ripple and transient response so it doesn't need to be too exact. Larger = less ripple but slower response. The feedforward capacitor is the one I would be more worried about. I suppose you could just plot out the output voltage vs feedforward capacitor and interpolate the curve.
The capacitances for tantalum and electrolytic are different because the capacitors have different frequency characteristics such as ESR, impedance at your target frequency, and bulk capacitance.
For example, if your primary aim for a particular ESR one type of capacitor might be able to achieve that at a lower capacitance than another type.
If your primary aim is to achieve a particular impedance at a particular frequency, that again would result in different capacitance between capacitors of different types.