A4 roll-folded to fit a DL envelope is a popular leaflet format. It’s not enough to divide an A4 landscape page into equal 99mm segments, because the ‘flap’ needs to be slightly smaller to tuck neatly inside the cover. On page 1 (the outside), the ‘flap’ will be on the left hand side, while on page 2 (the inside), the flap will be on the right.
It helps to make a ‘dummy’. To do this, save the file to your desktop, print it out, cut out both sides, stick them together and fold along the lines. Contact me if you’d like further help.