The 26th Founding was in 738.M41 and the first tyrannic war was in 745.M41.

What would be the status of a 26th founding chapter in the year 745?
I'm not sure whether a founding details when the creation of a marine chapter was authorised or completed - so would a chapter be at full strength by 745, or still in it's infancy?

If it is still in its infancy, what level of battle readiness might it be at? 2 or 3 companies maybe?

It's for some fluff I'm working on - my DIY chapter were created just prior to the first tyrannic war and were thusly almost wiped out by said gribblies - so hopefully I might be able to squeeze in their creation twixt those two dates.