The firfst layer height is different from what @Venom was asking. Z Offset can not be amended or changed during print. If you want to add more sqish onto the plate you need to amend the StartCode like this (which I have extracted from the amended StartCode available with printables.
;===== for Textured PEI Plate, High Temp Plate and Wham Bam Plate ===============
;curr_bed_type={curr_bed_type}
{if curr_bed_type==“Textured PEI Plate”}
G29.1 Z-0.04 ;squish of -0.02mm for Textured PEI Plate
{elsif curr_bed_type==“High Temp Plate”}
G29.1 Z0.03 ;raise of 0.03mm for High Temp or Wham Bam Plate
{endif}
With this there will be 0.04 lower offset i.e. more squish, when using the textured plate. Likewise 0.03 higher offset when using the high temp plate. On the fly change not poissible
Flow rate, same thing, nada.