The actual cache memory used is 50% of whatever memorysize you specify allocated to video and a smaller amount allocated to audio. Of the 50% allocated to video, three quarters is used for read-ahead caching, with the rest presumably for skipping backwards. (Based on reading the code.)
On the basis of experiments I performed a few months ago, I’d recommend a readfactor closer to 16, since that seems to perform better with higher-bitrate videos.