I can confirm the fermenter - semifluid generator ratio is 5:3, using the fluid heat generator as heat source for the fermenter.
Math
The fermenter needs 8000 hU/run, producing 200mB/run of Biogas.
Fluid heat generator provides 32hU/tick (with 0.5mB of biogas/tick), so the fermenter needs 8000/32=250 ticks/run and produces 200/250=0.8mB/tick of Biogas
The net biogas production is then 0.3mB/tick of Biogas , whilst the semifluid generator uses 0.5mB/tick of biogas resulting in the 5:3 rate mentioned above.
I've been using this in my world and biogas stayed stable, so I guess it's right.
However electric heat generator will consume more energy than you produce, as each fermenter running with 32hU/t provides biogas equivalent to 25.6EU/t, and the electric heat generator would consume 32 EU/t (not tested, but math is meth)