Is there such a thing as an "educated wild ass guess"? My estimate was grounded a bit in some T5 statements about the non-human population of the Imperium. I can't remember if the figure came from T5 itself, an Imperiallines issues, or Agent of the Imperium, but somewhere there's a statement that something like 50% of the Imperial population is non-human.
Traveller assumes that "life is everywhere," and T5 further assumes that sophontic life is far more common than anything we have ever seen depicted in canon. Like, multiple worlds per subsector harboring Native Intelligent Life.
The Imperium seems open to granting citizenship to any culture able to pledge fealty to the Emperor, no matter how weird. The Aslan are probably much less open: while they will accept any people who assimilate into their culture, they are also fierce cultural chauvinists. (Remember, the whole Cultural Purge was because they felt many biological Aslan weren't Aslan enough!)
The lower limit for the non-Aslan population is something more than 0%, as a handful of these non-Aslan races have been mentioned in canon. I think I went with 5%, which was approximately the number currently shown in Traveller Map data.
To get to 25% I assumed that (1) Many sophont species are not currently reflected in Traveller Map data, (2) these species were initially distributed relatively evenly across Charted Space, (3) those species who could/would not assimilate were either interdicted, exterminated, or driven out, resulting in (4) the Hierate having a much lower percentage of non-Aslan species than the Imperium.
So if the lower limit for non-Aslan was 5% and the upper limit was 50%, with no good reason to go higher or lower I think I just arbitrarily landed on 25% as somewhere in the middle.