If the raw capacity is 512GB, my statement still applies.  There is cache used for meta data in the Symmetrix.  The amount of meta data used depends on the amount of storage capacity configured in the system.  What is left over for user data is your "available cache slots"  The maximum write pending slots is a percentage of the total user cache, usually 80%, but in 75 code is now 75%.  The device write pending limit is set to 5% of the writable cache.   The DA write pending is obsolete, and even when it was used, was very rarely invoked.  The write pending limits are the point where any host writes must wait for a slot to be destaged before a new write can be allowed into the array.
how the slots are divided. If it is based on cache size, what is the standard slot size.
what is write pending slot in DMX?