Battlefleet Koronus tackles the problem of the amount of officers/warrant officers compared to the rest of the crew. But it says nothing about the members of the adeptus mechanicus, how many are there and what levels of competence/knowledge do they possess.

Today with my GM we tried to find a percentage of mechanicus personnel compared to the rest of the crew and reached around 1% (assuming that 10% of this 1% would be filled with higher tech-priests and the rest by tech-wrights and the like).

But here's my problem, I'm not really certain about it. On one hand I can imagine a small transport with only 18 full Tech Priests, but I have my doubts if 134 will be enough for a grand cruiser.

Any ideas?

Those numbers sound about right to me. If you really want a few more, maybe 1.5-2%. Anywhere in the 1-2% range should be good,there will be plenty of variance between ships.

Yeah, those numbers sound good to me, too. Bear in mind that the Tech Priests are the elite engineering crew: they will effectively have control over hundreds of ratings who will perform lesser tasks throughout the ship. To some extent they're more like consultants than crewmembers. They don't need to be a particularly large percentage of the crew.