I’m trying to point out to you that des_a[1] and -des_a[0] aren’t angles.
To get an angle approximation you need a ratio of two lengths, as computed in acc2rot.
What you have looks to me like 2 arbitrarily large or small numbers. In that case you most likely will compute either a rotation close to zero or a rotation close to 90 degrees.
So your “approximation” is probably very very very wrong, isn’t it ?