Here is the mental framework that I use to reason about PETG: In a nutshell you want to avoid nozzle contact.
Unlike most other plastics, PETG sticks to hot brass really well and every time the nozzle moves through material it will pick up some of it. Material around the the nozzle then sticks to a random place creating a blob. It can also cook, turn transparent brown and drop into the print. Investing in a plated nozzle or silicone socks helps but doesn't eliminate the problem completely.
Now to the questions.
1) Nozzle Distance
Distance to the plate has to be such that the plastic is laid down precisely without the nozzle dragging through the material (remember, avoid nozzle contact). Precise lines require the build plate to be level and the flow perfectly calibrated. If nozzle is too low and/or the layer is over-extruded then PETG will stick to the nozzle and rip the lines off the plate again. Inspecting the first layer is required for best results. I like to print a layer test pattern after the flow has been calibrated and tweak Z offset in 0.02mm increments until it's perfect.
With many other plastics it's ok to have a large amount of "squish" in the first layer as it helps to work around minor leveling issues. This is where the cookie-cutter recommendation to raise the nozzle when printing with PETG is coming from.
2) Extrusion percentage
Flow has to be near perfect. Down to one percent perfect. Even a slightest over extrusion and some of the excess material will end up on the nozzle when it makes the next pass. Under extrusion isn't great either as this can lead to holes and affect overhangs where thinner strands of a previous pass may not be enough for the next line to stick to.
There are two critical parameters: diameter of the filament and extrusion multiplier. This is how to determine the settings:
- Measure filament diameter. I use an average of ten measurements over about a meter (yard) of filament taken in multiple orientations.
- Calibrate the extrusion multiplier using a method described in Prusa
I print a 40x40x40 cube in vase mode with extrusion multiplier set
to 1 and fixed extrusion width (e.g. 0.45mm), measure the wall
thickness in three spots on every side, average the results and
compute the correction factor.
I perform flow calibration for every new roll of filament.