I don't think there is much penalty for a saddle that is too wide by 10 or 15 mm. I like a wide saddle generally but it has to be narrow just ahead of the sitbones. Knee over the pedal spindale is a starting point. From there I would try sliding the saddle front and back 4-5 mm or so for a few rides at each setting. Mark the saddle rails with a felt tip marker to keep track of settings.