well, i'm getting pretty decent 1st layers it seems and i did another one of those hollow cube prints with 2 .4mm walls and getting great width measurements with it. here's what i did. first started at extrusion multiplier of 1.10 and turned off my extruder esteps i had in the start gcode. then i printed that calibration cube and got wall width measurements of .91 - .92 ish. so based on calculation my new extrusion multiplier is .967 so i'll round that up to .97. anyway, the cube wall measurements are now .80 - .81. plus the height of that hollow cube is very nearly perfect 15.0mm. yeah, dpending on where i measure it ranges from 14.98mm to 15.12ish. but much better than before. also, i wonder if that could be due to mesh bed leveling - i changed my fade height in marlin from 10 to 20 to average out more at a finer gradation of height.
ok so my cube was great, now the first layer test i made .2mm high with .2mm layer and 1st layer of .2mm and % line width and % height both set at 100, the first layer test is pretty OK, it's like a piece of paper, all one piece and it measures in various places .20mm to .23mm so awesome!
i have only recently started playing with % 1st layer height and % 1st layer line width, but i also wondered about that - it shouldn't really be necessary and if it is, then the height really should only be the thing to change if you're doing say .12mm layers, but then the 1st layer height should be an even multiple of the actual printing height.
damn, i'm all excited now... it's working great and i don'thave to worry about esteps! i can just use that multiplier for extrusion!