For UK two year courses with placement, is the total stay-back period 3.5 years including student visa and post-study work visa?

Yes, approximately. If your course duration is 2 years, and you then receive a 2-year Graduate Route visa under current rules, your total time in the UK could be around 4 years including studies and post-study work. If the post-study visa changes to 18 months in future ((expected to change from 01 Jan 2027), the total would be around 3.5 years.