Split flow measured at the Exhaust is wrong.
When the OPTIC-4 is in Split mode (method is running) it is possible to measure the Spilt flow at the Exhaust on the back of the controller (done with an external flow meter). If there is no leak, the measured Split flow is the same as the setting in the method. Please be aware that there can be a small difference depending of the external flow meter. The OPTIC-4 flow is done with Mass flow control, not volume flow control.
When the OPTIC-4 is in Splitless mode (method is running) the measured flow at the Exhaust is not zero. The OPTIC-4 gas control has a Split/Splitless valve with a so called by-pass flow. If the OPTIC-4 is in Splitless mode the measured flow at the Exhaust is not zero but you measure the by-pass flow. This by-pass flow is used to have a stable pressure in the OPTIC inlet. (fast regulation)
For OPTIC-3 it is the same.