Good afternoon. I am part way through my Nitrogen pressure test. If it holds at 300 overnight I will proceed to the vacuum stage and then add R410a. I have a scale and all the proper tools. 

I started with a 50 ft coil of line and I cut off exactly 56 1/2 inches leaving a remaining line length of 45.29 feet (45 feet 3½ inches). Any length of line above 25ft requires additional refrigerant. Thus, I need to run a correction factor. I read the Mitsubishi installation manual. The formula is below:

Calculation: X oz. = 1.08/5 oz./ft. × (Refrigerant piping length (ft.) – 25)

So, for 45.29 ft the calculation is such:
X oz. = 1.08/5 x (45.29 – 25)
X oz. = 1.08/5 x (20.29 ft)
X oz = 21.91 / 5
X 0z = 4.382 

Does this look correct? The reason I ask is because I was watching a video on YouTube and maybe I misunderstood it, but it made it sound like I needed to add refrigerant for the total additional length added. In other words, I would need to add essentially double the above refrigerant because total line length is 45.29 for both legs – or about 90 feet total. 

I think my instincts is correct and I just need to add the 4.382 ounces and call it a day. But I wanted to get other opinions.

Thanks!