Serge,
There are probably other threads in more detail about FG knots but here is my 2 cents, based on my experience.
I too learnt from Duncan's video, but since then I do things a bit differently. The main thing is that ALL of the half hitches are pulled up very tight. I don't wrap the tag end around my little finger like Duncan does, as it starts to hurt after a few minutes, and I don't feel like I can get my wraps tight enough.
Instead, I do two wraps using his method around the leader, then I carefully grab each strand of braid, pinch the hitches down so it doesn't slip off the leader, and pull both strands very tight. I hold the scrap end of the leader (the bit I am melting) between my teeth for security when pulling the braid tight.
I repeat this after every 2 hitches, until I have done about 20 or so up the leader. Then I do alternating half hitches around the leader and main braid line, again, but I pull each one up very tight.
I then carefully melt the end of the leader into a neat little ball. After that I do about 12 half hitches up the main line, to protect it from the melte end of the leader and also to reduce the hinging effect a bit.
Each knot takes me about 7 or 8 minutes to tie. A good one will look like this:
Note how tight the wraps should be, not only around the leader, but also butted up against each other. When I pull them tight, I make sure they snug up against the previous wraps so there is no leader showing at all between the hitches.
Sometimes after casting one for a long time, I get a little bit of separation between the melted leader end and the hitches up the braid. I don't like the look of it much but I have caught multiple big GTs on knots like this, so it is not the end of the world. Retie it if you have doubts!
I have a lot of faith in my FGs and once had an almighty struggle busting off a snagged stickbait in 10m of water. In the end the 130lb leader broke before the FG, which was done in 80lb braid. It showed no signs of pulling apart either.