the best guess is to take the ecl of the creature they have and subtract the ecl of theit nomrally. I don't have the book here to do that. However, last time it was brought up the ecl looked really low.
The mindflayer has an ECL of 10 accroding to the ECL calculator I use on the internet. The official mindflayer ECL is proably differnet.
This makes the paragon templates ECL 16. Which is definitely a little low. I'd make it at least equal to its max CR boost (so I'd make it at least ECL 18).
The offical illithid level adjustment is +18, and the paragon illithid level adjustment is +26. That means that the paragon temple is level adjustment +8. (I suspect they no longer use that illithid LA.)
ECL +8 is far too low for the paragon template. I'd stick it at ECL +16 minimum, especially for the games (epic-level, I presume) in which you're likely to use it.