If you used the fuse rating to determine the wattage, use the fuse that is on the 240V input of the transformer.  If there is no fuse on the 240V input, then you need to multiply the transformer output fuse current rating by the transformer output voltage - but I would guess that there are multiple taps and multiple fuses on the transformer output, if so it gets messy.


WARNING:
Nothing wrong with over specifying the power rating, only a problem when you under specify.  If you under specify the transformer power, it will fail or worse catch fire.  The fuses would offer no protection for an underrated transformer, as they would be rated for the correct power rating.