no need to time it or guess, the downtime effect only happens once you get a strat under 50% ...
while it is true that 2 skilled B29 pilots can flatten the city, theres still no effect unless you also get a strat down under 50%. so figure 4 skilled pilots to have any meaningful effect on just one strat factory. darbar from 4 sets of buffs heading to HQ will almost certainly get a response from the rocketeers, which realistically means you need to double the buffs (for bomber losses and abandoned runs, those pesky 163s always seem to time their attacks for your drop 
).
figure 6-8 formations to have a decent effect on 1 or 2 strats then, 8-12 if they cant all precision bomb from >20k.
It is this kind of post I *really* wish someone in the know from HTC would comment on and make a general statement such as "the damage done to the factories is directly related to the length of time in which the OBJ on the bases are repaired". I dont expect an exact formula as to how the factories and city effect the resupply of the fields, but some sort of dangling carrot would be nice.
I've put a stop watch to how long a city and factory stays down, and how much longer it takes a field to resupply itself via convoy, and I say there is a correlation, but just how much I do not know. I wasnt able to get the exact ratios to match.
FWIW, a single flight of B29's can take down the city to at least %50 if all of the 40/500 lb bombs are precisely dropped. The secret it to get the center bomb to land between to city blocks, that puts one each 500 lb on either side. So with 3/500 bombs can take out 2 city blocks. If the bomber pilots is REAL good (lucky), he can get 4 city blocks if the bomb drop is dead center of the 4 way intersection (I've got lucky twice). Remember, I was able to get the city down to %51 and the ammo factory down to the mid-%50's with 1 load of 40/500lbs. So if I can get the city down to %51 with 30-ish 500lb bombs, so can a lot of other bomber pilots. Looshy is correct in his 6 hours of repair time for the city, the factories repair themselves faster but how so I'm not sure. Also, I can not pin point the [damage to city/factory = increase in field repair time] formula. But there is a correlation and I don't believe that %50 is the threshold.
