In the Files area under "Methods" there is an Excel spreadsheet 
named "Facing Curve".  The cells under the "L" column have equations 
in them to calculate arc facings.

I started with the equation of a circle:
(R-F)^2+(M-L)^2=R^2

R is the radius
F is the feeler gage
M is where the flat table ends from the tip
L is gage length from the tip

This equation works if all the measurement are in the same units.  
Conversion factors are needed to use inches with mm with 2*mm gages.

The equation in the spreadsheet is an algebraic and triganometric 
manipulation to get:
L=M-(R*sin(acos((R-F)/R)))*25.4*2

Here R and M are in inches and L and M are in 2*mm glass gage 
readings.

I adjust M and R until I get the desired tip opening (at F=tip, L=tip 
rail thickness) and facing length (at F=.0015", L=facing length).  
For those of you good at spreadsheets, the solver tool can do the 
iteration needed.

I know the above may be too much for many.  It is something I 
developed for myself that is quite useful.  I'll try to explain the 
confusing parts better if you ask questions.