Whats the heter

Daf Hashavua

Gittin 6b

וא"ר יצחק שתים כותבין שלש אין כותבין במתניתא תנא שלש כותבין—

R. Yitzchok said two words may be written withoutשרטוט   (engraved lines).

three words may not be written without שרטוט .

A line that isn’t straight and is curvy going up and down would

probably  be considered a non kosher שרטוט. It probably needs to

be a straight line.

Many Ketores written on a Klaf have   added to them Tehillim Kapital 67

למנצח בנגינת in the shape of a Menorah*. The lines are a semi circle shape.

Just wondering what’s the Heter.