Ian, for all of Efke 25's unforgiving qualities, reciprocity is not one of them. Andrew O'Neill did his own tests and had the following, which I found is correct with my equipment. Thanks Andrew.
1 second on meter = +1/6 stop
10 seconds on meter = +1/3 stop
100 seconds on meter = +2/3 stop
Matt, remember Francesco is using BTZS times for development, so his SBR numbers have a lot to do with expanding and contracting development along with exposure variations, bellows extension and any filters he may be using. This may be why your numbers look to be different.
Perhaps the Professor of 8X10 can give us a short lesson on Efke 100 reciprocity?