Spool on some mono backing first of a higher test than your main line. Then crank on a 150 yard spool of Fireline in whatever test you use.
You’ll need to leave room for the Fireline.
I use a very small barrel swivel to attach the two lines.
The key is to put exactly the same amount of backing on so both of the reels are calibrated the same.
The key to line counters is that they must have the same amount of line so they read the same.
When finished tie the line to a fixed object and walk back 100 feet to see if they read the same or if it is even 100 feet. If they are off, mark them somehow you know which one is less accurate and by how much.
After some useage, respool the fireline when there is a differnece in the amount on the spool so you’ll have equal readings.
If you have more than two linecounters pair them up according to similar counter readings.
The biggest advatantage of line counters is being able to replicate the previous presentation.
The trollers bible is just a start and you need to adgust from there to get your lires to run where you want or think they are.