Disc 300tdi auto box oil filling from replacement

Hi guys, can anyone help? I know this seems like old ground but I cannot find any info on filling after replacement, I know it holds 9.2 lt from empty but only get 8 lt in before over filling comes out of the breather, gearbox replacement was a mammoth task and I don't want to have to repeat it again, before I do any more, do I drain down and feed in slowly when engine is running, or do I crack the oil cooler pipes to bleed the system,:confused