Solved Custom G-Code?

The only thing I've failed to work out for my printer is the custom G-Code. If anyone can edit the default to my needs, I'd be very grateful. I'm looking for code that can do everything that default code does but at some point after heating the extruder it extrudes 20 mm of filament. 20 mm that is later retracted in the end G-Code, essentially stopping me from having to hand-extrude filament until it's right in the hot end.

 

G1 F200 E20 ;extrude 20mm of filament at a feedrate per minute of 200
G1 E-20 F300 ;retract 20mm of filament at a feedrate per minute of 300

Retraction can be faster than extrusion