I would use ASA 25 for the negative...but since life is not that simple, I would also add a half-stop of light to the meter reading since the actual ASA is also determined by one's shutter, consistancy of the strobe output, age of the Type 55, one's mental state and who knows what else! So for a one-shot only, I'd rather risk a slight over-exposure than being under-exposed.

Vaughn