The pre-post prices listed in form guides 24 hours before a race day are often wrong. A better guide that I often use is the tipsters panel in these papers. If you allocate 3,2 and 1 points to their first, second and third selections then you generally end up with a better approximation of the eventual SP. A possible reason is that the tipsters are a better reflection of market opinion than the individual who frames the pre-post.