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I think you're aiming at the wrong target. Your math assumes that the display's light output is totally linear, or 1:1, which it likely isn't.

The % stim is only the signal level sent from the source to the display, and can range from negative numbers to over 100% on some test disks. 1% is 1% regardless of how bright the display shows it. The same goes for 5%, 10%, or any other percent stimulus you output. How much luminance is actually produced is a function of the display's capabilities and how it is adjusted.

Correct gamma will be the greatest help in not crushing blacks. Instead of trying to achieve a target contrast ratio, focus on correctly setting black level and getting the best fit for your chosen gamma curve. The contrast ratio will most likely be nowhere near the 100K:1 you want, but you'll have the best picture your display can produce.