I don’t think you can use the pixel clock. You need the exact horizontal line timings to calculate the PAR.
Take the NES for example. Here are the timings: https://wiki.nesdev.com/w/index.php/NTSC_video
The active video of the NES is not just 256 but actually 256+15+11+1=283 pixels because of the left&right borders. So a CRT would display the 283×240 resolution in 4:3.
Scaling this to a 4:3 resolution like 320×240 means you have a ratio of 320/283. 320:283 is 8:7.075. Hey that ratio is familiar 😀