as much as our brains are designed to understand logarithmic scales, this percentage problem never really feels intuitive for me. If you have 1 part U235 and 99 parts U238, to get to 2%, it is 50.5% (50/99) of the work to get to to 100% enriched. If all you need to get to weapon grade uranium is 90% enriched, you need to eliminate 98.88888 of the 99 units of U238 (1/1.11111 = .90). So there is no real significant difference in your progress, you are now 50.56% of the way there.