We look for 59 Brix at 209 F. The hydrometer comes in handy. It's best to favor the bottom of red line if it's at the top you may come up short of the 66.9 to 68.9 Brix needed after it's cooled to 60 degrees.
This is when the first filtering occurs. We used to go by temperature but now we just have an eye for it. From here the syrup will be reduced further on the stove. Remember to use double filter!