You have to know what you want to accomplish first. If you don;t know what voltage you want at a point, we cannot calculate a resistor to make it.
It is all a matter of Ohm's Law. All the current through the dropping resistors goes somewhere. Almost all of it through the tubes. SO each triode, how much current? My general rule of thumb for crude estimates is 1ma per triode. A dual triode then draws 2ma if its halves share B+ node. SO start with a voltage at some point you want to drop from. Calculate with Ohm's Law what resistance at 2ma drops the desirted amount of volts.
Remember to that all current downstream flows through the up stream too. In other words, all the small tubes plus the power tube screen currents flow through your choke. Now the screens branch off, and all the small tube currents are left. If there is more than one stage of preamp B+, then keep in mind that the lowest voltage tube currents are sharing a path with the higher voltage stages currents.