Ideal capacity is a complicated concept, its not just how many times a year do you use all or part of the extension versus its cost, its also about being able to satisfy demand for the biggest games and keep the 'wants to attend' fanbase active and engaged, with a ripple effect on other matches. Unless you are a small club, in a period of anything but disaster putting in extra seats inevitably raises the 'normal' attendance a bit and the payback period may be over a huge number of years. Generally, if a club 'thinks' it may make sense, then it probably does. We went from 42 to 48 and it was undoubtedly the correct decision IMO, we need to be back in the top flight to go further, but if someone else fancies part funding it then its a no brainer IMO.