So meter back in to the pack is the only way to do that providing the charger can do it. I believe that charger can, it's just a balance of cost over benefit, though convenience is the primary benefit with that charger.
Every choice is a compromise.
So the SOC/Ah/fuel gauge is a solid concept? I believe you mentioned something similar earlier in the thread, the one linked above will run off the pack voltage so as such will be a load across the whole pack, and not contribute to unbalancing the pack?
No. You can do it once. Take say 70Ah out, replace 60Ah. Next day you can only take some lesser amount out, and return even less Ah.
You may do it twice maybe, but will need to put back less each time as you no longer know properly where you are or how out of balance.
And you can only do this over a short time period because you dont know which cell group is getting out of step. Through temp difference, and self discharge differences between groups.
I may add say 30Ah back after removing 40, as measured with a simple Turnigy watt meter. But I will only do that while in my van as I drive for eg. But since the hobby charger plugs into the same van connector just as easily, I bought recently a third one... It is safer and can give a full charge/balance. Allows me to be sure, and top up and balance.
15Ah is too small , and is subject to heavy load , when it is full . 
