PETG filament doesn't stick reliably to fake Buildtak surface
I have printed kilometers of PETG and found the sweet-spot for my brand to be 240 &deg;C for the hotend and 70 &deg;C for the build plate (for my Ultimaker 3 that is, the extruder temp is 5 &deg;C higher for my home build HyperCube Evolution). The reason for the 70 &deg;C is that the glass temperature of PETG is around 70 &deg;C. The PETG is flexible at that temperature such that there are no stresses because of shrinkage causing the PETG to keep attached to the heat bed surface (aluminium, glass, Buildtak, etc.). A little PVA based glue (stick) or spray (hair or specific print sprays) can even further improve the adhesion. A slow first layer also helps adhering better.

Note that the hotend temperature should be calibrated to the speed you are printing. If you print faster, a higher hotend tempearture is required. To determine the sweet spot for your filament you can print typical calibration towers that can be found on e.g. Thingiverse. Note that you need to manually change the G-code file after slicing of the tower or use plugins of your slicer to change the temperature at a certain level.

Furthermore, PETG does not like to be cooled by the print fan, so keep cooling fan rpm low to prevent layers not to bond (else you get a sort of string cheese print).



Edit:

I use parametric stair case style calibration prints that include the slicer print settings that are to determine the best settings for temperature, print cooling, layer size and print speed.