If the rate is discounted for the first year, the CPT will be different during the discount period and the follow-on period. Ditto fixed rates. The APR attempts to show the long-term effect of these fluctuations. Which is why it's not suitable for calculating the CPT.
If the nominal rate is not quoted on a lender's website, you need to ask them what it is before you can calculate the CPT.
If the rate is discounted for the first year, the CPT will be different during the discount period and the follow-on period. Ditto fixed rates. The APR attempts to show the long-term effect of these fluctuations. Which is why it's not suitable for calculating the CPT.
If the nominal rate is not quoted on a lender's website, you need to ask them what it is before you can calculate the CPT.