Neither A, B not C catch the important points although C get nearest.

Let's only look at the first centrifuge at a point in time when it is working (i.e. it has swallowed 40 bright U235 and 5 dark U238) AND has additional 4 U 235 and 12 (a full swing of the stack inserter) U 238 in its input slots. When the fifth cycle completes the following happens:

- The first inserter (ejector) puts the two dark U238 onto the outer lane of the belt. These are ignored and queue up and are transferred onto the inner lane.
- The ejector swings three times and puts 3*12=36 pieces of the bright U235 onto the belt.
- The injector takes these and puts them into the input slots with three swings also.
- The input slots now contains 36 new U235 and 4 older ones and 12 U238 and hence the next cycle starts. Therefore all 40 U235 and 5 U238 are swallowed leaving 12-5=7 U238 in the input slots.
- Both input slots can not accept more items. The inserter will first pick from the inner lane and hence adds 12 more U238 (total: 19 U238)
- While the inserter does this swing the ejector will put the remaining U235 from the output slot onto the belt. Because it was the fifth cycle an additional U235 has been generated due to the productivity modules. Therefore this last swing will put 6 more U235 onto the belt. These will pass the inserter which is still feeding U238.

- Dark U 238 is only picked if there are less then 10 in the input slot. But if so a full load of 12 U238 is added so the maximum is 9+12 = 21 U238 in the input slot. In this case two cycles can start without feeding U238 and in these two cycles the inserter will also grab the remaining pieces of U235 and put them into the centrifuge. This increase the maximum overfill by at most 3 pieces U235. This will not accumulate however because the ejector can transfer up to 12 U235 onto the belt while the inserter is busy with U238.
- The following centrifuges work the same but are offered additional U235 from "upstream" which they will grab if they can. But even the next 5 centrifuges will
*not*fill up to ~80 pieces of U235 as they get less from the previous centrifuges than they spit out -- on average.

So I think that this setup is ideal for bootstrapping because it is quite simple and the first centrifuges barely overfill so the next ones can start working earlier. When 6 ore more centrifuges are working then increased overfill is usually not a problem anymore.