Start from rich main needle and start leaning it out while doing WOT runs.
When the engine begins to make good power and rpm the changes on the needle won't make too much difference in the engines power.STOP.
For a noob i would go 1/8th of a turn richer from the above point to be safe.

Listen to other engines at the track...you will be able to tell the difference between a lean motor and a fine tuned one.
After i'm done with my tuning i take some temps from the engine...if they look good then i'm done for that day-time.
If it's hot i'll open the main needle a bit.