>>I'm starting to contemplate my layout power requirements, and I need a
sanity check (quiet out there).<<
>>I've laid out the booster positions so that no track
point is more than 40' from a booster/PM42 location. In a couple of
cases, one booster/PM42 feeds three levels of track, which encompass
four differ mainline sections; in other cases, a Yard on one level
takes two PM42 sections, while trackage above and below use the other
two. PM42s will not be used for reversing, just power management.<<
1) It sounds like you are determining booster positions and quantities based on a maximum bus length or 40'. The number of boosters, with boosters being expensive items, is usually determined by determining the total power load of the of the layout (how many engines running at the same time, plus passenger car and caboose lighting) and then dividing by the amperage capacity of the booster (usually 5 amps for a N or HO layout). Having done that, you can define your booster districts so no booster is expected to have more than a 4 amp load.
2) You want to maintain a max length of bus runs of 40' - that's a reasonable upper limit. But, don't forget, you can split the booster output at the booster and run 40' (your designated bus length) in _each_ direction for 40'. Beyond that, bus lengths can be extended further if the bus pairs are twisted (3-5 turns/foot), and R/C "filters" are used at the ends of the buses.
>>All that being said, I find that I only have one location where I can
justify an 8A booster; the rest will happily be managed by 5A boosters.<<
Conventional wisdom suggests that you limit yourself to 5 Amp boosters. In a short situation the section of track served by that booster could potentially receive the entire 8 amps of current, and that's a hefty amount and could lead to damage to equipment. Far better to use two 5 amp boosters in lieu of the 8 amp.
>>So I guess I'm asking, have others encountered this? In my case, my
max-bus-length rules force me into far more booster/PM42 combos than I
might otherwise purchase, given my expected track loading.<<
As I mentioned above, consider splitting the bus at the booster and run 40' in each direction, and create booster areas where the _expected_ max current draw on each 5 amp booster is between 3 and 4 amps - that leaves you a bit of ceiling room for the occasional extra current draw.
I'm headed out on a long trip either Saturday or Sunday, and don't know yet what e-mail access I'll have on the way. If/when I do, I'll check in to see if you have any followup questions.
Others will also most likely step in to assist you, too.