Set flashmeter to ISO 100 and retract the dome or use flat disc.

Time 1/30th (irrelevant but you have to set something).

Try to get the flashmeter to read f/16.0... That will give you same as an EG&G.

Now you want to reduce the intensity because that's too much for testing ASA 400 film.

I have a 1.84 filter, and I also add .60 on top of that.

So reduce the intensity of the flash or add filters to add up to density 2.44 (give or take 0.30).

Is that about 8 stops? If so, you could try for f/1.0 at ISO 100

If you have to use a filter, a physical filter is preferred over a neutral density filter. For example sheet metal with holes drilled or slots cut in it, placed over the front of the flash.