"So according to Mersenne the soundpost is most often placed directly under the foot of the bridge, which if carried out to a tee, puts the soundpost fair and square in the middle of the soundpost plate, the place where one would expect it to stand."
Heale, Michael. "Viol Soundposts." Early Music, 9(4) 1981 :521-524.
This is exactly what I have done.