Just found this handy tool that tells you when a player's value is set to rise/fall
http://www.fiso.co.uk/crackthecode.php
It explains the method on there somewhere, but the percentages depend on whether the NTI (net transfers in) has reached the Target NTI. Those over 100% are set to rise 0.1m (Mata, Aguero, Mignolet, Ba, Simpson, Yaya Toure (unfortunately) and Graham tonight) whilst those below -100% are set to drop 0.1m.