Thank you for contacting Intel Communities.
In this case, the SSD that you will use for acceleration mustn't contain any OS after accelerating the system, and any remaining space on the SSD may be used for data storage only.
Hope this information helps.
Please don't hesitate in replying to this post in case you need further assistance.
I've got exactly same intentions, got the same drive (but z170 and 6700K) and want to use about 200GB for Win 10 and remain 40GB for HDD cache. And I guess that it is really often concept of usage. There really no solution for use SRT for this purposes? Why this limitation about containing OS? For modern SSD it is shouldn't be a problem.
An SSD that is being used for acceleration shouldn't contain any OS, in case that there is any remaining free space, it can be used to store personal data.
The reason why any OS shouldn't be installed in the SSD is because of Intel SMART Response Technology limitations.