For a detailed description of the PTAL methodology, see the ‘Assessing transport connectivity in London’ document at the following link:
https://content.tfl.gov.uk/connectivity-assessment-guide.pdf
The link to calculate your PTAL score is:
https://tfl.gov.uk/info-for/urban-planning-and-construction/planning-with-webcat/webcat
UK NC2014 Technical manual - hyperlinks to be updated accordingly in next re-issue.