Focus not even across the frame

I have 90 degree camera. Managed to focus it somehow good at the cented of the frame but it goes out of focus by much near the edges of the frame /gradually/.
Is this normal? It makes lense callibration at positions of the card not in the center big challange - I can’t get less than 0.40 /plus or minus/ when the card is not on center position and on position 1 /center one/ I get 0.20 or less easy

It is normal - another user here made themselves a helper for this:

Exactly what I did. Will post image tomorrow