You’re misunderstanding my entire premise.
When we calculate the destruction of a mountain, we have to calculate its volume before calculating the destruction.
To do that we make reasonable assumptions about the mountain’s size. For example, we assume a height of 609 meters because that is the minimum irl mountain height.
The we proceed to pixel scale the mountain’s dimensions based around it’s assumed height, and calculate a volume.
From the calculated volume we calculate the destruction based on the energy required to destroy mountain rock per volume.
We must calculate an initial parameter based on reasonable assumptions (mountains in the work of fiction or at least as tall as the smallest mountains irl), before we calculate the actual destruction. In simple terms, we calculate 2 things to get our end result. One initial calculation based on reasonable assumptions, and then another calculation without any assumptions.
Now let me outline the perception blitz case.
Let’s say we are given the fact that a character can move at 100 m/s, and another character blitzes them over a know distance. Furthermore, throughout the work of fiction let’s say the 100 m/s character has displayed the ability to consistently react at those speeds within melee range, melee range being within arm’s reach for the most part.
To obtain a time frame we must make a reasonable assumption. That being, because the character can consistently react at 100 m/s at very close ranges, they’d be able to react to that speed over the distance of 1 meter. Why is that reasonable? In reality melee range would be even shorter than 1 meter, as most human’s don’t have meter long arms.
Next we calculate the perception time based on the canon fact that the character can move at 100 m/s and react at those speeds over short distances, with the reasonable assumption that short distance in the context of melee combat is 1 meter.
From then we can measure the distance the blitzing character moved and divide by the time to get their speed.
Do you see how in both instances we make a single reasonable assumption to calculate an initial parameter (volume for mountain destruction and time for speed blitz), and then use the calculated parameter to calculate the value of interest (destruction of the mountain and speed of the blitz)?
They are in no way, shape, or form any different whatsoever. So, unless you want to argue that we nuke all destruction feats where we have to assume a size of the object being destroyed, the instance I outlined is not calc stacking if we don’t considering calculating mountain destruction calc stacking.
Regarding “parameters that don’t change”, arguing mountain sizes are a single constant uniformly across the board is also objectively wrong. We quite literal fix mountain sizes to be constant based on an assumption. However, they are not in fact constant in actuality.
Edit: I want to preface that I am not advocating we assume any character can react to stated speeds over 1 meter, only if the display the ability to consistently react to close quarters/melee range at the stated speed (e.g. the assumption is far from unfounded).