What should i select layer for fine-tuning on Xception?

I’ve tried to choose layer for fine-tuning on Xception.

I know that 1st conv layer of block is chosen.

In xception case, there are add layers like resnet.

ex) model.layers[114].name : block12_sepconv3_bn
    model.layers[115].name : add_11
    model.layers[116].name : block13_sepconv3_act

so, what should i select layer for fine-tuning?