The only thing that is to be taken care here is that code and xdata space do not collide and hence result in corruption by overwriting. When you build the firmware, various *.lst files are created for each of the source files (*.c) added to the project. Size of the code and xdata can be found by scrolling down to the end of the respective *.lst file. Using these values set the code and xdata range such that they do not collide.
The starting memory for xdata/code is application dependent as per the need of the design. Please note that specifying starting memory below 0x80 is not recommended. This is due to the fact that the interrupt vector table resides in memory locations below that.Please refer to the following link at keil website for more information on configuring BL51 settings: