First, the frame used:



Here are the relevant measurements [metric]:

frame [inside] - 231 x 502 mm
passe-partout cut-out - 120 x 170 mm
border between images - 33 mm
border left/right - 35 mm

I created a template using an A3+ sheet, which fits under the full passepartout though it will not cover the complete length, only 483 mm. Onto that sheet I put 3 templates [120 x 170 mm each], centered the middle one, set the other two such that they are 25,5 mm from their respective sides, centred top-bottom. Then saved the whole job to later replace the three templates by actual images.

Before doing a test print, did I miss anything, is my template sound? Any other ideas I should pursue, particularly when creating one for the 5-er RIBBA?

I have actually been using the full page editor; didn't find a way to measure the dividers. Thus I went by defining the edges [easy for the center piece].

Ah, misunderstanding. I did that, was thinking of some visual help, say, guidelines from/to the rulers.