Firstly, building cake is unnecessary. Just my opinion.
Secondly, the amount of dottle left depends on variables like how dry the tobacco is when you pack the bowl, did you pack loose or crammed it in, are you a puffing billy or a slow sipper ? Drying the tobacco and packing looser is the key. Just tamp occasionally if it feels like it is about to go out.
I often don't have enough time to dry the tobacco sufficiently (not enough forethought as to what I'll be smoking on any given day). Under those circumstances I will use a few meerschaum Chips or Nording Keystones Filters or a Sharrow Philtpad in the bottom of the bowl. I find it gives me a dry smoke and most bowls smoke down to ash even if I don't dry the tobacco. Some laugh at this practice and say it's totally unnecessary - I find it works for me, and saves wastage (given the price of tobacco in Oz, it adds up).
But, dry, dry, and dry your tobacco some more.
Once the bowl "sours", dump it. Trying to smoke dottle is plain unpleasant.