This is my first post on the forum. I am not sure if this is the right forum to post it but I guess this might be the right forum. Actually, I am learning to calculate Julian Date manually. I know, there are web sites on the internet that have online calculators but my idea is to learn and master the procedures instead of being concerned with just getting an answer.
Right now, I am consulting "Practical Astronomy with Your Calculator" by Peter Duffet-Smith (Third Edition) and on page 7 (scanned page attached) he has given the steps to arrive at the Julian date. While his steps are clear but I am basically stuck on the last step (#6) as I see a value (underlined with red marker) I am not sure how it was derived and there is no explanation.
Could someone please help me with this?