Non Residential CGT

Was trying to fill in NRCGT form on line. In the form you are expected to work out your own PRR or Private Residential Relief calculation. It asks you to work this out but then later provides 3 options for either a 2015 rebasing valuation( 2 methods) or the total period of ownership.I have owned the property since 1981 and been abroad since March 2015. If i opt for a 2015 valuation am I right in assuming the element of PRR is different so I have to go back in the form and change it? Help sheets and HMRC links are no help here for this general query. I am assuming PRR for a 2015 valuation is different than a 1982 or 1981 valuation date but HMRC notes do not explain this..

Hi Eamonn,
If using the rebasing, you can only apply Private Residence Relief from that date.
Thank you,

Many thanks for your reply. I think your form causes confusion though as it asks you to work out the PRR or Private Residential amount in pounds yourself BEFORE and then later in the form gives you 3 options with 2 valuation dates. It does not explain that if using 2015 rebasing valuation date the PRR element changes and you have to go back into the form and change it.It does not explain this anywhere. Because I had lived in the property for 81% of the time as my sole residence before it was sold I had thought that was the PRR element. I am only mentioning this as I think others might make the same error causing a lot of miscalculations but thanks for clarifying this issue.