Should I use given resolution or calculate resolution based on the max range and number of sig bits?
It's important to recognize the the resolution provided in the ARINC 429 spec is an approximation so you can't necessarily use that exactly as it is.
From the commentary in section 2.1.6 Data Standards

For convenience, all binary word ranges in
Attachment 2 are shown as whole binary numbers
rather than such numbers less one least significant
bit value. Also, the resolutions shown are
approximate only. Accurate resolutions can be
determined, if required, by reference to the range
values and numbers of significant bits for the words of
interest.

In order to provide a range of 1.5 to 20, the label will have an actual range of 0-32, which with 11 significant bits, will provide an actual resolution of $\frac{32}{2048} = 0.015625$ (approximated to 0.016 in the ARINC 429 spec)