I just had another play with different dioptre combinations, I have a 135mm Schneider lens so I thought I would have a look at a few wider combos, I know that for 4x5 the 35mm equivilent is 3 times smaller eg a 150mm lens is a 50mm lens equivilent on a 35mm camera.
I had a go with the +10 lens and added on a +1 to make it an 11 dioptre lens which works out at around 90mm focal distance, so a '30mm' lens, it works ok, fair bit of distortion and I needed to bring the bellows of the speed graphic in a bit as well. I then tried a +14 lens with a +10 and +4, making a 70mm lens or a '23mm' lens eg superwide. I had to bring the bellows all the way home but it would focus, its basically a fisheye lens! Very interesting effect though, would be interested to see that stopped down. That lens would have been a 70mm f1.3 lens which is rather cool, if I had used the 88mm filter it would have been a 70mm f0.8