I make a LOT of ginger beer using the ginger beer SCOBY. I can say that in my experience there is a very wide margin of appreciation regarding the amount of ginger (or lemon juice, etc) to add to a brew. I usually use a 5L demi-john. It might be filled to (say) 4L.
Two heaped desert spoons of ground dried ginger added to the initial brew will produce a "firey ginger mix". One dessert spoon will produce a "good warmth". A teaspoon of ginger powder in 5 L will produce a "back of the throat ginger tickle" - which many initially prefer, especially for new/young ginger beer. Obviously YMMV as the strength of any ginger is not standardised. But is it usually approximately the same.
However the brewing with SCOBY is an ongoing process, involving lots of fill and drain operations, sedimentation, occasional rinsing and so on. The way the brew is bottled might well affect the taste. For example the brew can be shaken to get the ginger into suspension/solution and then that can be strained into the final bottles (with ginger in suspension) for carbonation. That will produce a gritty gingery taste, usually with a lot of bottle sediment. If the brew is decanted from the settled ginger powder then the taste will be lighter (with no sediment).
I don't find it necessary to add a spoon (or any amount) of powdered ginger per day, in fact I find that makes the brew too string for most tastes. However I have noticed that added a table spoon of fresh ginger "matchstick slices" can add some variety to the flavours that is not just "ginger heat" but incorporates some hints of "fruit" as well. Some prefer adding raisins instead of sugar once per day, but I don't tend to use that method. It does work though but leaves a lot of rasins in the SCOBY.
Using fresh ginger can present some problems unless it is cut into suitably large slices. I say that because powder ginger is easy to rinse from the SCOBY using a fine sieve but ground fresh ginger is about the same size as the SCOBY grains and so stays with the grains and is difficult to separate. Using slices or "matchsticks" makes the rinsing/filtering much easier.
I prefer to add powdered ginger once at the start of the (say) 3 day brew. I add a desert spoon of table sugar once per day and give it all a shake. When I want to bottle some beer I allow the brew to settle and decant some of the liquid though a strainer into some PET plastic bottles. I add a desert spoon of sugar for each litre(quart?) bottle for carbonation. The drink is usually ready about 3 days after bottling and is best served cold. Depending on the circumstances people may prefer to use less (or more) carbonating sugar.
Like most ferments the drink is nice after a few days but, also like most ferments, it is even better after some time spent maturing in the bottle. Ginger beer matured for about 6 months is just wonderful. Soft and mellow with a final bite of ginger.
I would very much recommend using a big "mother" fermentation to contain the SCOBY and the initial brew - then transferring the liquid to the final bottles. I do not recommend fermenting with the SCOBY in the final delivery bottles - it just wastes SCOBY.
