Sigh...
It's there so that the engine doesn't melt when it gets shit fuel.
In fact that path has nothing to do with KFLBTS. KFLBTS is the base path. KFFDLBTS * DLBTS is the additional path which enriches with a delta lambda based on ignition angle efficiency.
Setting it to zero is stupid, as is tuning BTS without having EGT measurement, because you can never know how much it needs to not melt.
I really don't like your last sentence though.
"I don't get the point of X, it just complicates things".
With 91oct i have to run deep into 10/11 anyway, so BTS based on ignition angle isn't going to do me any good - at load there is never good angle efficiency, period. May as well just run pig rich all of the time at load or WOT. So for shit gas, you can, actually, just set to zero to simplify tuning.
In theory, once you have that set up, you can unwind it, but you'll NEVER be out of DLBTS anyway, so what's the point?
I know this isn't the "right" way to do things, but for a first pass and to get a feel for how BTS/LAMFA/ATR interact it isn't going to kill you if you have an aggressive BTS/LAMFA to start with.
If i run shit gas worse than 91oct, i go from shit 5 deg to shit 0 deg timing, upon which i can always enrich via KR.