Dear Gabriel,

I wrote an answer via APUG mail. A part of this mail may be of more general interest:
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It is quite easy to determine the Schwarzschild exponent for yourself.
- take a shot of a grey area (e.g. a greycard) with an exposure time of 1/4 s. No exposure failure is expected at this time, but we are close to the border-
- stop down 10 stops. If you don't have enough stops you may use a filter or wait for less light
- make an exposure series for that 10 stops less light on the film, starting with the 1024-fold exposure time, in half stops
- Process the film and look for the time which gives the same grey on the negative like the 1/4 s shot.
- The reciprocity failure is then 1 - (more stops needed with the long time) / 10 <- The 10 comes from using 10 stops less light.

Example
- starting exposure 1/4s with f/2
- stop down to f/22 (7 stops) and use an additional 8x grey filter. [Or simply use only 7 stops difference]
- The calculated exposure time is then 256s [32s for 7 stops]
- make a shots with 256, 362, 512, 724, 1024, 1448, 2048 s [32, 45, 64, 90, 128, 180, 256 s]
- look for the grey tone which is similar to the 1/4s, f/2. Let us assume it is 1024s.
- You need 2 stops more (256s -> 1024s), the rec. failure p is 1-(2/10) = 0.8 .
- The same for 7 stops would give around 90s for the equivalent grey tone, which is 1.5 stops more. p = 1- (1.5/7) = 0.78.