I found that the Tile Scale matters when printing poster-wise from Adobe. Firstly work out what page size it thinks it's printing (in my version this is under Document Properties).

If this is A3 then you would think that setting the Tile Scale to 50% would mean that you get two sheets of A4? Not quite. You have to factor in the Overlap (in inches) and possibly the page margin (which applies to the A4 sheet in this case, not the A3) - so - by trial and error it turns out the scaling that works is 48%.

To make things worse, if you press enter after giving it a new scale, it starts printing straight away, without showing you on the print dialogue what it is actually going to print. Grrrr...

However, you can get it to show you by changing the print from Portrait to Landscape or vice versa (It doesn't matter which, as the print dialogue isn't taking any notice of that). Hmmm....