I'm using a P6419/P68xx probe and it came with a thick and a thin elastomer. How do I know which one I should use?

I'm using a P6419/P68xx probe and it came with a thick and a thin elastomer. How do I know which one I should use?

If the PCB thickness is 1.27 mm to 2.36 mm (0.050 in to 0.093 in), use the nut bar with the thin elastomer.
If the PCB thickness is 2.36 mm to 3.81 mm (0.093 in to 0.150 in), use the press-in nut with the thick elastomer. 

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