I did a little research and apparently 20% will make a low grade bomb. I was recalling that we use 95%, not that you have to have it to create a mess.
Without going into a lot (nor a little!) of classified details, you can make quite a bit of damage (particularly with the “dirty leftovers/spread radiation around) type of bomb at low enrichments ... IF you are willing to transport by truck or train or ship (yacht or fishing boat) or 8 foot shipping container.
The very highly enriched U235 that we used above Hiroshima is essential for small air-dropped bombs, but if you accept “lots” of wasted (non-reacting) U235 around the smaller bit that does react fast enough to blow up, then a lower enrichment works OK. More expensive, but they have lots of oil money to spend, and lots of hatred to spend it upon.
Everything nuclear gets much, much less expensive anyway if you are willing to work quickly and contaminate your buildings and people, instead of trying to keep them safe and clean for many years.
Obviously, the Iranians, in their hatred, can go for the cheap and fast and contaminated solution. Part of the “cheap” solution is to create much more of the lower enriched U235 mix. My equations have NOT covered other materials (some would contaminate the reaction, some contribute to the nuclear reactions inside the bomb’s core) that could be present. Leftover Pu239 for example, Thorium, etc. from the original enrichment reactions.