isolated flyback feedback
Hi Berni,
I guess your problem is linearity, not to keep forward current constant. This is why you search for an tight input-output spec, right? ACPL-M43T is an digital coupler rather than analog (OK it is open collector output, but CTR is loose).
I would recommend a high linear optocoupler like HCNR201 from Avago. CTR is much tighter. Moreover on the primary side there is an error feedback photodiode for further correction.
Datasheet:
Hope it helps.