I am looking at regular fares Melksham to Swindon (as a worked example) now that the Flexi Season tickets are with us.  They are priced at £79.20 for eight days of travel within 28, versus a weekly season that offers travel for 7 days contiguous days at £44.90 (realistically used for 5 return trips) or daily returns at £10.80 (up to the 07:53 departure) or £8.00 (later trains, anytime at the weekend).  Annual season ticket - £1796.00; monthly £172.50.

The Season Ticket Calculator on the National Rail website has been upgraded / updated - not sure where all the facilities are now.  It has been enhanced to add various options and let you add in the number of days a week you're likely to be travelling, and as far as I can see it assumes anytime (i.e. peak) travel, and lacks an option in trying out daily fares for those with senior or disabled railcards who - at least outside the South East - may do rather better buying daily tickets than season tickets in some situations.

https://www.nationalrail.co.uk/times_fares/Season-Calculator.aspx

For adults travelling 8 or more times per month on the 05:33, 07:21 or 07:53 on a Monday to Friday, one of these tickets makes sense. If the travel rate exceeds 17 journeys per month, a season ticket is recommended.   In between, consider buying another 8 out of 28 ticket and transfer to the second one when you have used up the first - an assumption here that you'll be carrying on the same work pattern the following month.  Holidays get to be interesting calculations, and if you have a railcard that's valid at peak times (senior, disabled) you'll do better buying tickets for each day.