From the manual.

Place GeoLink at a flat, aligned horizontal surface anywhere at your model in order to get a precise reference. (for example the Spirit unit)

My model has raked landing gear for extra tail blade clearance. Therefore when sitting on the skis the fbl is angled down towards the nose.

Is it still OK to set geolink on top, or do I need a perfect bubble level and shim gear to precise level?

Thank you.

usually I use the "zero temporarily" button

put geolink on top of the motor ( if the motor is upside and big enough like in a 600-700 heli ) or on top of the spirit unit
then press "zero temporarily" on geolink screen
then you mount the geolink trying to align it with the 0° / 0° shown by the program

this way you will be sure that the geolink will be aligned with the 0° of the helicopter

Thank You for confirming. I just wanted to make sure it was level with heli and not true bubble level.
I did read the temporary set in the manual.