I simply took the pure usable area and I come to 81m2 on the ground floor
because we don’t pay for living space but for the usable area
If you lump all room floor areas together, that might be about right, I would have estimated around 78 sqm. Regardless of which standard or version you apply, there are partial creditings of areas under sloped roofs in height classes – so with the rough formula, the term "one-and-a-half-storey" is quite appropriate. At least without a knee wall, it almost fits.
and I want to be at 130m2,
The question is which rooms of what sizes are needed, and you must not forget hallways and stairs. And if you simply add up all areas equally, that also applies to utility rooms and storage rooms.
Whether you will hit the target with 130 sqm, however, can only be poorly verified with unsuitable floor plans.
For the price, it ultimately comes down to the total area, i.e., whether more knee wall means less slab foundation area required would be roughly neutral.