OK, I did some more testing. Up until 1/25s it stays at roughly 10ms before the second curtain (then at about 75% of track length).
For 1/30s shutter speed and faster, setting second curtain sync does nothing but the normal first curtain sync is used. The change at 1/30s is documented in the manual of the 70D (it is also in the manual of the 5D3).
If they implemented this limitation, the 10ms I'm seeing might be normal, but should in any case be documented, as this drastically reduces the usefulness of the second curtain sync for any shutter speeds faster than about 1s.
First curtain sync flash seems to happen about 2ms after the curtain opens at the bottom of the frame (nearly at the middle of the 4ms track) and nearly at the start of the exposure at the top of the frame. This means that the shutter takes about 2ms to fully open (starting at the bottom of the frame) and afterwards the flash has about 2ms to fire until the shutter closes again (starting at the bottom of the frame).