Your on the right track . Measure out a fixed distance and check to see if it's reading correctly . If you line counter is reading high add line , if it's reading low remove line . I usually start my spools off reading a foot or two low...meaning they are slightly overspooled . That way as line settles into the spool or I cut off to retie I'm still good . When it comes time to respond I strip off 300 ft of line and tie a splice knot of some kind and top the spool off using the other line as backing .