What do you mean thickness of wall the bridge??
When going from inserts to no inserts you have to compensate for the thickness of the insert wall.
With inserts the cut to cut measurement for the holes is slightly shorter than your actual span because the hole the insert goes into is bigger than what it would be if it was just a fingerhole without a insert.
When the insert is put into the hole, the wall thickness of the insert added to the shorter cut to cut measurement will then equal your actual span.
Also if you use inserts with either 1/8-1/4 built in forward pitch (Powerlifts) and go to no inserts, you have to add in the pitch.