Calculate garden house roofing

Our development plan stipulates that ancillary structures must be covered with the same material as the main house. So, tiles on the house mean tiles on the roof. I have also found a company that sells kits that can support a tiled roof. For various Braas roof tiles, they also provide material lists so that one can procure the roofing materials themselves. However, our house will have Creaton Magnum tiles, which naturally look somewhat different. The closest to these would be the Harzer Pfanne, also wavy but with shorter intervals. If I want to use the Magnums, I would have to calculate the material list myself (rather simple) and also adjust the roof overhangs accordingly (not so simple?). Has anyone among you ever calculated something like this and can tell me if I am basically calculating correctly?

First, the basic approach:
Cover width is 296mm, the GH is 4m long plus overhangs. One verge tile covers 250mm and I need two of those. Then I would have 14 tiles in between, resulting in roof overhangs of about 25cm at the back and 40cm at the front.

And now the area calculations:
Width of the GH is 3.5m. At the ridge 7cm clearance, at the eaves 39cm, between those you are allowed to cover 39-41cm. The roof has a pitch of 18 degrees, so for 1.75m per half I have to calculate about 2.1m. That fits exactly 4 intermediate rows at 41cm. This results in 200cm width, equal to a 25cm roof overhang.

Here again is the calculation as it comes from my Excel

Is that correct or do I have a mistake in my thinking somewhere?