there is a poorly named function for this

fixed_wide/src/fixed_wide_traits.rs

@@ -63,7 +63,7 @@ impl<const CHUNKS:usize,Frac:Unsigned> Fixed<CHUNKS,Frac>
 		//2. divide by 2 via >>1 (sqrt-ish)
 		//3. add on fractional offset
 		//Voila
-		let used_bits=CHUNKS as i32*64-1-Frac::I32-self.bits.leading_zeros() as i32;
+		let used_bits=self.bits.bits() as i32-1-Frac::I32;
 		let max_shift=((used_bits>>1)+Frac::I32) as u32;
 		let mut result=Self::ZERO;