While I am familiar with Leon Todd, I am not sure which video in particular you are referring to, so I'm hesitant to comment on what he is doing, especially versus what you're doing. Leon Todd is one of the "master" tone smiths around here. What ever he is doing must be good for his guitar and the tone that he's looking for.
As to whether 20% versus 90% is reasonable, I would say that it's possible. It is a big spread, but different pickups can have significantly different output, even just comparing passive pickups. I have a Knaggs Kenai with Alan Price P51 pickups installed. It is noticeably lower output than a second Knaggs that I have with Seymour Duncans installed.
What I do is set the instrument input sensitivity using the hotest guitar pickups that I will be using for a gig. If I am using a guitar with lower output pickups and the lower output pickups don't have enough oomph to them, then I'll adjust the "Input Trim" in the amp block to compensate for reduced pickup output.
The term "tickling the red" leaves room for experimentation. It's kind of a cliche around here, but it really is the best approach - "use your ears". If it sounds good to you then you don't have to follow anyone else's rules. The main thing that you want to avoid is turning the input sensitivity up too high so that you are clipping the input sensitivity. This type of clipping produces a nasty digital harshness that sounds terrible. To get a sense of what this sounds like turn up sensitivity until you hear what I'm talking about (make sure that you turn the amplifier volume WAY down first!). Then back down the sensitivity until this sound goes away.
Experimentation is your friend. Make sure that the volume is down on whatever you use to amplify your Fractal. Start with very low volume or you can damage speakers. Increase volume gradually with input sensitivity set really high to hear the digital clipping I'm talking about. Then reduce input sensitivity to taste. As long as you start with low volume you won't break anything. The Fractal units are built to be flexible and you can't break the Fractal using the internal settings. The complexity can seem intimidating, but part of what makes Fractal products so awesome is that there is so much that you can do with them.
One other thought is to do a search on Youtube on "Fractal Audio Gain staging". I can't think of a video to direct you to, but I know that there are forum members who have done videos on this. Perhaps someone else who reads this thread may have a suggestion on a good gain staging video.