After a long long spell of being away from fishing, i'm about to start up
again so this may seem a daft question;

how far up the spool depth should i go to maximum fill?

I'm using a fixed spool with new line on and realise too much on the spool
can almost be as bad as too little.

I've loaded it on until the line is flush with the back end (or bottom) of
the spool with approx 2-3mm 'under' the front end (the face).

Jo