That’s not how that would work. You would have to use the explosion of the Long Memory itself, not a separate calc of Monstra’s KE. Otherwise it’s just calc stacking I believe. That’s like me using a calc for All Might’s durability to calculate someone else’s durability because they took a punch from him.
She didn’t physically tear through him, she just blasted through his body with magic. And the damage she did to him was absolutely minuscule given how large he is, so scaling her fully to Monstra based off that wouldn’t make any sense.
That hole is what, at most a 10 meter diameter? Monstra is a, as calculated, nearly 9 kilometers big whale. She is absolutely not fully scaling as if she could blast apart the entirety of Monstra in a single attack with her Base magical beams, or scaling physically to Monstra.
This is like having a parasite in your body burrow a hole through your intestines to your stomach in an instant. Obviously it will hurt, but the parasite is not as powerful as a human and would get squished. Small creatures inside a big creature don’t typically scale as if they’re the same power as the big creature just because they can hurt it from the inside.