"- I haven't actually counted but where I've read up to in chapter 34 I'm pretty sure its at least less than half and feels closer to a third... "
I have now, quickly and roughly, counted the Japan to isekai amount.
Out of about 873 pages in 34 chapters (as far as I've read as of this posting), 344 of them take place in Japan, 529 in the isekai world (due to the speed at which I counted and possible misclicks/counts I give these numbers an accuracy to within about 10).
This means there are about 1.53 isekai pages to every Japan page.
In the first half the balance was pretty even going back and forth for about the same time between each world. Japan had some lead early on in the back and forth but then isekai took the lead more often, but enough to get caught up to in a couple Japan centric chapters. But with the build up to and now in the labyrinth arc its become weighted much more towards the isekai world.
Once this arc is over they might have a lot more Japan to balance it back out, but as of now its less "Elf in Japan" than the title might suggest.