TXP320 Reciprocity

Anyone have a good formula for figureing out long exposure on TXP320 (4x5) I got some really heavy ND filters, and want to use them to simulate effects seen due to the multi-minute exposures of night photography, in open sun.

How different does the 320 vary from the 400 anyways? Think I could get away with using the 400TX number from the (there was a url link here which no longer exists) without disastrous results?

IIRC, the old versions of TRI-X 400 and 320 had the same correction according to Kodak. They were both much greater than any corrections Howard Bond found for most recent films. The old film would have had a coefficient of 1, so the time to be added would simply be Tm^1.62, where Tm is the measured or otherwise estimated time if there were reciprocal trade agreement. I think that would be a conservative estimate. Anyway, overexposure is hard to come by in dim light. If the first coefficient were only 0.5, you would be off by one stop using 1.0.