Megan says it is 80% of GDP
(kind of). Her mental models of understanding sovereign debt dynamics however, are wrong, not because she's dumb, but because the standard models we all carry around for this stuff is incorrect.
A country is not like a household, in that it can issue it's own currency and that it can force that currency to be accepted and used because it has the power to tax.
However, a household can replicate this if parents issue scrip in payment for chores, but also demand scrip in exchange for privileges (like allowances, later curfews, etc.) In such a situation, is there a "tipping point" for parental scrip? Is there some ratio after which the parent has just issues too much scrip and now will need to start borrowing scrip from other parents in order to pay it all back?
Of course not. If a parent accidentally over issues scrip, then the children, with excess scrip, will bid up the price for whatever scarce good they want which they can buy in scrip. Back in the real world, this is called inflation, and yes national debt levels can get high enough to trigger inflation.
But inflation is only triggered when the children actually start bidding up prices in scrip. If the children just save the scrip, then there is no inflation. So "excess" scrip is measured not in %s, but in the psychological impact it has in scrip users. You only know when you have "excess" when you start seeing inflation, and kids can decide to spend scrip at any point.
Parents don't need scrip from other parents unless they want to hire other people's children, and those other children want to buy things only available in their parents scrip.