In connection with what Rabbi Michael Tzadok wrote about sqeezing in ג"פ אשר before והיה אם שמע, I recall doing it once without a problem, but that was pre-digital years so I have no picture of it. Therefore today during a break I scribbled it again and presto!! I even had 5 mm spare...
I wrote this on a line size that is equal to a 15 cm מזוזה and the width of the line is the "standard" 12.5 cm. In the beginning I really wrote tight but toward the last words I saw that there is enough space so I stopped squeezing and there is still a 5 mm "extra".
I never really understood the logic in "starting under this word" or "starting under that letter", all that has to be done is to measure the space accordingly.
And if there still is not enough space you can also simply ask the משרטט to give you an extra 5-6 mm ( if you have access to him).