FF4 SNES SRM Checksum

By changing one value at a time in Zyrthofar's tool (ie: Max HP from 1 to 2), I noticed that both bytes of the checksum independently increment by the same amount.  For instance, if I increase Cecil's Max HP field by 1, then both checksum bytes increase by 1.  If I increase another field by 2, then both checksum bytes increase by 2.  They seem to be somewhat independent of each other.  

I don't know what the final two bytes (or depending on endianness the 3rd and 4th bytes from the end) are 0xE4 and 0x1B.  Interestingly these add up to 0xFF, which I could see factoring into some sort of checksum.  They could also just as well be an 'end of slot' marker.  The 5th byte from the end also changes - maybe as part of the checksum calculation.  I haven't figured that out yet.

It'd be great if you could take a look at it.  Maybe it's a really easy problem and I've just overlooked the obvious solution?