I believe the 20% threshold was chosen because that represents the largest plant and processing cost step in enrichment.<p>In other words once it is at 20% you can use a much smaller enrichment plant to take it to weapons level due to the nature of the cascade.  My back of the envelope calcu-guess is that 20% enriched represents about 95% of the work on the way to weapons grade. [edit: guess based on only isotope ratios, not any specifics of yield of the various enrichment processes]