Does the basement rule include that the floor area can be smaller with built basements? If not yet, then that would mean it might even end up costing the same in the end. Or is that already factored into the 3/4?
The costs of a usable basement under the entire ground floor (corresponding to the floor slab area considered) are compared with the measures to "avoid a basement even though the plot requires it." "Usable basement" here means a non-residential basement (regardless of whether it is used for anything other than storage) of the simplest standards with surface-mounted electrical wiring, etc. The effect that using this basement for residential purposes saves or compensates above-ground rooms to a corresponding extent is indeed correct, but that is another matter. The basement rule only deals with the question of whether, in the specific example, simplified as "L-blocks instead of basement rooms," this is economically reasonable or a fool’s errand. Although even the simplest storage basement would have more than two meters of story height, the price parity effect with two meters of height difference is already fulfilled 100%. One and a half meters is three quarters of two meters, hence the corresponding value in the example. I can hardly believe myself how accurately such a rough flat estimate works as a prediction. Similarly, a residential basement compensates 1:1 above-ground living space.
In terms of your initial question, the "moral of the story" is that with one and a half meters of height difference under the house floor area, you would do well to build a full basement and use it mixed with living and storage rooms. Using it only as living space (hypothetically, since you also need storage) would require open excavations due to the lighting of rooms with normal sill-height windows, which would then make the affected basement living spaces somewhat more expensive than their above-ground savings. Hence my advice for mixed use. At two meters of height difference, even "empty" basement rooms would be economically "free," while at only one meter, calculating a partial basement could already be worthwhile.
Digging the house in so far that you would then have to partly "re-excavate" it is basically as foolish as raising it up and "flattening" the plot. The degree to which it is strategic or nonsense is told to you by the basement rule.