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by the way, an f/160 camera needs exposure 100 x that of an f/16 meter reading or estimate.
You could shift the decimal place over two positions, then do whatever quick reciprocity failure correction you chose, if you aren't the preprinted-chart kind of guy.
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Converting f-stops is relatively easy... 
Lets say you have your "regular" camera or light meter with you and meter the scene as f-16 1/125th. And you know your pinhole is f-180.
To get a conversion factor in time for your exposure (Fstop_pinhole / Fstop_meter) ^ 2
(180 / 16) ^ 2 = 127 (rounding up from 126.5625, close enough)
Multiply the exposure time by 127
127 * 1/125 = 1.016, call it 1 second. Now apply any filter factors, then finally apply the reciprocity factor correction. Murry gave you a reasonable way to guess at the factor in the absence of any other information.
Simple, isn't it? What I would do is make yourself a chart based on the good-ole Sunny-16 rule and then scale it for -1, +1, +2 and +4 stops (snow/sand, hazybright, distinct shadow/backlight, kinda gloomy) and pre-figure in the reciprocity failures. Then keep these charts tucked in your camera bag or just taped to the camera!
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FWIW, when your box of Velvia is done, maybe try the Provia, which has excellent reciprocity characteristics. Maybe the Velvia does, too, but I have not tried it.
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