I need to create a planar inductor with a specific value of 28 nH on a 4-layer PCB. How do I determine the proper trace geometry?
I love to use the Saturn PCB toolkit for this kind of calculations nightmare. It does everything ! (except the coffee.)

Try it. For your inductor (28nH) I get roughly 


3 turns 
0.2mm conductor spacing 
0.2mm conductor width
3.320mm outer diameter
1.320mm inner diameter


=>28.08nH

I tried to play a little bit with conductors width and spacing value, and from what I see the inductor will not be that sensible to manufacturing defect. For example, reducing the trace width to 0.14mm and raising the spacing to 0.32mm still gives a 26.1nH impedance, which is 'in tolerance'.

But, these kind of components are like PCB antennas and should be tweaked. You can add additional 'feeding' traces to the "prototypes designs" around the first spires and cut them until you come close to the required value (best RLC Q), or scratch the soldermask and solder a wire, or...

Keep in mind planar inductors are cheap in the long run (for big series of cheap product) but cost a lot of time during design due to tuning nightmare and tolerances checking craziness.

But imho it's doable !