One fermenter supplied with 16 MJ/t running on fertilizer can serve over 4 stills at 5 MJ/t each, given enough saplings. Math points to almost exactly 5 stills, but someone answered somewhere, that reality won't keep up perfectly with these numbers.
Let's assume 4.5 stills can be fed by one fermenter, then this one fermenter can keep 5 36 HP boilers running (if they are already on max temp).
One still needs 250 seconds for 3 buckets of biofuel, so 83.333 seconds per bucket.
One fully heated 36 HP boiler runs for 101.010101 seconds per bucket.
101.010101/83.3333 = 40/33 = 1.212121 (boilers server per still), so you need 5/(40/33) = 33/8 = 4+1/8 stills to serve 5 boilers.
A tree farm serving enough saplings to maximize a fermenter can without problems serve at last one another 36 HP via their logs (posted that design elsewhere). I assume, that you can even feed at least one other boiler with that, too (and having a bunch of charcoal as a byproduct).
Just tested:
One max speed fermenter (using water, fertilizer and saplings) can keep 5 stills running with a very small reserve.
With the above number of 1.212121 boilers served per still you can now keep 6 36 HP boilers running (when they have reached 1000°C) with one fermenter (5*1.212121=6.060606).
Another idea to boost biomass output:
Normally the production of apples will be too low to only use apple juice instead of water. But with the following contraption you could boost production occasionally, so your saplings will last a bit longer.
Squeezer->Tank (small tower of BC Tanks). Cobblestone Pipe with Gate (IF tank full THEN redstone on), feeding into a RS Latch. Another Cobblestone Pipe with a Gate (If tank empty THEN redstone on) feeds into the other side of the RS Latch. Your Fermenter is filled via Liquiducts, one input is the BC Tank, the other an Aqueous Accumulator. The Latch output that is on when the Tank is full, deactivates the AqAcc and activates the suction of the Liquiduct on the BC Tank. The Liquiduct will run dry on water after a short while and juice will fill it instead. When the fermenter runs dry, the juice will start to pour in. When the Tank becomes empty, the suction will stop and the AqAcc will restart, filling the duct and the fermenter again with water, as soon as the juice has been used up. The cycle starts anew.
Another thing: 7 Industrial Steam Engines (the 8 MJ/t ones) can feed both the fermenter/still setup and also the fertilizer production (cobble pulverized to sand (has an exclusive pulverizer), crafted to sandstone, pulverized to sand and nitor, the nitor pulverized to saltpeter which is crafted with sand to fertilizer). This should produce a little bit more MJ than needed for the two, so the squeezer could maybe run on that overproduction.
All in all:
One Steve's Cart tree farm feeds 6 36 HP boilers with biofuel, at least one with the logs (most probably even two or more (untested)) and produces charcoal to boot. Net gain should be nicely over 1000 MJ/t. Now convert all this in magma crucibles to lava, burn it in GregTech thermal generators and watch your happy matterfab.
