Tuning flow rate to adjust first layer squish

[blastrock]

Hello!

I have a question regarding the first layer squish adjustment. I didn't spend too much time tuning it on my printer, but had good enough first layers, then I wanted to improve that. Btw, note that my bed is a textured PEI sheet, this probably had an influence on my process below.

My intuition tells me that we shouldn't adjust the first layer squish using the z-offset. I started printing a calibration cube, and measuring it gave me 30.2mm. My conclusion was that my z axis was 0.2mm too high, so I added 0.2 to the zoffset.

Obviously, I tried printing a first layer test and it came out pretty bad, very rough. So I thought, my z offset is good, my calibration cube is now 30.02mm, what's not good is that there's too much plastic in the first layer. So I started printing a first layer test and tuned the flow rate during the print (as you suggest to do with the z offset). I started at 100% flow rate and went down, I ended up at 60% which gave me a very clean first layer.

So I have two questions now:

1. You don't seem to talk about dimensional accuracy in your first layer squish guide. Your method does sacrifice it, no? If I am saying something stupid here, please tell me. Maybe this deserves to be explained in the guide too.
2. Does my calibration method make sense, to get both good first layers and good height accuracy?

Also, prusa slicer does not support setting a flow rate for the first layer only, so I took inspiration from the solution described here:

;BEFORE_LAYER_CHANGE
;[layer_z]
G92 E0.0

; set the flow for the 1st layer to 60% of extrusion multiplier
M221 S{if layer_num==0}{extrusion_multiplier[0] * 100 * 0.60}
{else}{extrusion_multiplier[0] * 100}
{endif}