I've recently made two wide-ish Roman blinds using an online tutorial.
The heading (top) allowance will depend on the size of the headrail used, mine was 7cm; the hem (bottom) allowance was recommended at 9cm. Add this to your desired finished blind length and you will get the 'cut fabric drop'. For exampe, if the finished blind length is 180cm, cut fabric drop = 180cm + 7cm (headrail allowance) + 9cm (hem allowance) = 196cm.
As your window is wider than the fabric, you're correct thinking that you would need about 2 lengths, but the actual calculation needs to be adjusted to include a whole number of pattern repearts to allow for a nice alignment of fabric panels. Using the above example for the cut fabric drop calculation and the pattern repeat of 109.09cm from you link: 196 / 109.09 = 1.8 repeats, rounded up to the whole number = 2 pattern repeats; adjusted cut drop = 109.09cm 2 repeats = 218.18cm*.
Finally, to estimate the actual amount of fabric you need to buy, add another pattern repeat to accomodate a nice placement of the pattern on the blind, so for this exmaple, 218.18cm + 109.09cm = 327.27cm.
However, most online shops will only sell fabric by a whole metre, so you will have to buy 4 metres.
On top of that, you will need lining fabric (plain, blackout, or thermal), headrail (all-in-one cassete systems are much easier to use than old school wooden battens and cords, but more expensive; and I'd recommend a geared chain mechanism as the blind of that size would be quite heavy), rods, bottom rail bar, little rings and/or breakouts, some buckram and velcro tape.
Please also bear in mind that at 240cm it will be a very wide blind. If you're planning to make it yourself, it would be quite a feat. Mine were just under 155cm wide and I struggled, as I didn't have a table/surface large enought to keep the whole project flat, which is important to make sure the final product looks neat.