A recent alternative to the Thaum as a unit for measuring magic. It's like the difference between Fahrenheit and Centigrade/Celsius (there's also Kelvin, but then it starts to really confuse people). An intellectual awareness that 20ºC are about right for comfort doesn't stop you knowing, deep down where it matters, that 75ºF is the proper temperature for a warm summer's day. And as for 297 K.... that might well be another way of quantifying a warm summer's day, but the size of the number suggests a warm summer's day on, say, Mercury.
The Prime, numerated in powers of ten so that it multiplies into hectoprimes and kiloprimes and divides into centiprimes or milliprimes, may be a far more logical and mathematical unit of magic, but it's never going to stop older wizards asking "so what's that in old money, then?" or dividing on their fingers to convert it into Thaums.
The Thaum represented the amount amount of magic required to create one small white pigeon or three billiard balls, causing dispute over how small a pigeon and factions arguing for snooker balls and alchemical rather than ivory. The Prime is more reliably measured, being the magical energy required to move one pound of lead one foot. (Similar to the Roundworld newton which is the amount of net force required to accelerate a mass of one kilogram at a rate of one meter per second squared.)