Derek
I'm afraid that I am having difficulty explaining my concern.
If you look at the chair seat from above, there will be no problem with the curvature in the y axis. However if there is a significant concavity in the y axis I wonder if the flat surface in front of blade will not allow a proper shaving to be made. Of course once you use it that may not be a concern.
I found that adding some curvature ahead of the blade in two planes made the tools more useful.
front view.jpgside view.jpgi
I hope this makes sense. Let me know how things work out
Bill