[SkyMarshal]

>or comparing the cost of servicing the debt per year to GDP (so debt servicing cost climb to x percent of gdp).

Yeah, or yearly debt servicing cost vs yearly tax revenue, or delta debt servicing cost vs delta tax revenue.

The GDP component only hints at the underlying issues - what it costs us to service the debt, and what it would cost us to stop adding to it, and what it would cost us to pay it down the hard way.