Spreading the writing of your 6 bytes across the whole FLASH sector is indeed a good idea. FLASH endurance is a measure of how many cycles a page can go through programming and subsequent page erase cycle.

For setting the options on memory size and so forth, in the Keil IDE, use the dialog accessed through the Project/OptionsForTarget menu item. Selecting the specific device (on the Device tab) will select default settings. You can see specify to use those defaults on the Target tab with the two check boxes. You can then elect to modify the default settings on the BL51 Tab. Change the Code Range down by an even number of pages and then later when your program grows too bit the linker will be able to give you a warning that the code is too big for the space alloted to it.

Michael Karas