Hi Thanks for your question.
The 1-6 vs mark tells you that either 1 or 6 cylinder is ready to be set.
To know which stroke you are on make sure whatever cylinder you are ready to adjust that the rockers are loose on that cylinder and on the other cylinder they are tight.
So when your on 1-6 mark and number one is ready to be set the rockers are loose on 1 and tight on 6.
The 1-6 marks are on inside of accessory drive pulley and the pointer is on front cover you just line them up,,make sure you turn in normal engine rotation,,a 1 and 5/16" socket goes on pulley nut and you turn just like you are trying to tighten nut,,if you go past mark turn engine back 1/4 turn then back the correct way to mark.
I always like to start on number 1 but you can start at any other mark.
One other note: most times the numbers are covered with paint or rust, use emery paper to clean,,it also has a line between numbers that you align with pointer.
questions or problems let me know 'm happy to assist