Design a Photodiode TIA for a certain bandwidth?
At some point, you apply the theory, and use an adjustable component to achieve a desired result (especially when you're running out of time). A test jig with swept frequency response, or transient rise-time measurement will be required. Few variable capacitors can accommodate your target of 0.5 pf. Stray capacitance variations will likely dominate your calculation efforts anyway.
Your desired bandwidth is less than that of your circuit, so if you wish to limit bandwidth with this capacitor, its value will be higher than 0.5pf.
In this kind of circuit, a small variable capacitor can be constructed from two insulated wires (teflon coated wires would be good):

Capacitance is adjusted by clipping off sections of the open end, being careful to ensure the open wire ends do not short together. Of course, you can reduce its capacitance by clipping, but you cannot increase capacitance. At 25 MHz., twisted length should be a small fraction of a wavelength, so transmission-line effects should be insignificant, and you have mostly a capacitive reactance between wire ends. Just keep it away from other components.

You'll still have to decide if stray reactance variability or temperature effects will detract too much from your desired result. And if this is a product, is the cost of a test jig, and adjustment procedure worth it?