That's a good start but you need to account for the rsi*8 increment. Since this uses two registers I would just use a push/pop and lea on another register to get the final address:
I suggest you do better work at it and start learning how to debug/back-trace. Your instruction is "mov [rcx+rsi*8+00000728],r14". Which means one of the registers is a base address, the other is a structure offset. Say your address is 400000. And it's in "rcx". Then your offset to the position is 0 and it's in "rsi". So you get 400000+0*8+728. Now if the offset changes, and instead of a 0, you have 1, then you get 400000+1*8+728. See the dynamics behind it? That spot of yours is recipe for failure. Am assuming game's Watch Dogs, based on the Engine name (Disrupt).
If they found the opcode through 'see what accesses this address' they could check what rsi is equal to and do a compare to always grab the right address though right?