I apologise in advance, this post is mostly unnecessary rambling, but feel free to read it anyway.
I know many of the people on this forum are at best apprehensive about the
Greco-Numeric assignment for describing psyker potential - although you may remember that PrecinctOmega suggested some improvements on the last 'Clave, which Ynek sums up
here.
But nonetheless, I've been contemplating it and what it might actually mean.
One idea that came to me is that it doesn't actually describe power potential, but
rarity.
Much like IQ only describes the likelihood of an individual having a given "intelligence" (let's not get into an argument about how much use IQ actually is), rather than being a number that is directly descriptive of their brainpower, perhaps the Assignment does the same.
If we start with Rho being typical, and work up the scale with each subsequent category being five times rarer than the last (although not necessarily five times more powerful), then... well, I don't really know what that exactly means, but it makes it a bit more than just a completely vague number.
I'm using five because it roughly translates to the roughly one in a billion rarity given to power levels that exceed Delta. It consequently gives a one in 400,000 rarity for power levels above Iota (where conscious control starts), which doesn't seem too implausible to me.
You can do something similar with the negative assignment levels, although those will need a much greater rarity between levels - the number 30 seems to create reasonably believable numbers.
While the rarity of a given power level can ultimately be tracked back to the power of a psyker, the idea of a logarithmic rarity scale can explain why there isn't necessarily a "top end" to the scale.
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And yeah, I know I'm trying to put numbers on something which wasn't ever designed to take them, but it's just an idea that struck me and made me think "Hmm, that might even make sense".