Lift Z before Skirt print

Hi all

i have two problems and i do not get an answer on google

1.
before start of print i use this start G code

G1 Z10 F5000 ; lift nozzle
G28 ; home all axes
G29
G1 Z10 F5000 ; lift nozzle
G28 X0

then the Z lowers to 0.2
after that it goes to pint the skirt ...etc

i want if it is possible not to edit the code manual every time

Run the start G code
get to Z 10
go to the start position of the skirt
lower the Z to 0.2

2.
every time at the end of print i want the extruder to retract 20mm
every time at the start of the program i want the extruder to extrude 21mm
everything is ok

but if something goes wrong i need to manually retract the fillament 20mm
that is why i had created a macro button on the pronterface

G92 E0
G1 E-20.00 F300.00000
G92 E0

but it allays extract more and i i start the print partial of the first layer is not printed
what i do rong??

Kind regards all

1 Your G28 X0 clears the auto bed leveling you did. Change it to G1 X0. For the other change you need to edit the gcode, or write a post processor or make a custom version of slic3r.

2 Use the gcode to change to relative moves.

oh thank you for the answer

1. the G28 is new added, just a test. you the only way to change the G code is with a post processor i will see what i can do
2. if i place G91 before the fillament retract and after G90 will work?

G92 E0
G91
G1 E-20.0 F300
G90

this change must be in the pronterface buttons and in the slic3r Start and end G-code with the change from - to +

THANK YOU