Programming the L9201 Engrave Cycle can be frustrating due to the fact, depending on what copy of the Fadal manual your using, it can be somewhat misleading.  So lets write some L9201 code...
 

Must have tool dia in Tool Table (DT)

R2 = Full height of desired letters

Do not use Start/End spacing

Do not adjust your letter positioning using the Emulator graphics

 
Actual Engraving Length
(Note in this example we are using the letters; F & L)

F = .9118

L = .8941

Height - Tool dia x Actual Engraving Length
 

Letter height is 1.

Tool dia is .125

(1. - .125) x (.9118 + .8941)
         .875 x 1.8059 = 1.5802
 

 
Centering the letters on the part
(the center of the arc in our example is X 4. ,Y 3.)

X
1.5802 / 2 = .7901 (1/2 letter width)
4. - .7901 = 3.2099

Y
3. - .5 = 2.5 (1/2 letter height)
2.5 + .0625 = 2.5625 (1/2 of tools dia)
 
 

 
Here is the finished code for our example . . .

N1 G90 G94 G80 G70 G40 G17
N2 M6 T1
N3 M3 S3055
N4 G0 X3.2099 Y2.5625 * Start Location
N5 G0 Z0.1 H1 M8
N6 L9201 R+0.1 R1+0 R2+1. R3+0. Z-0.007 F12. (LF
N7 G0 Z.1
N8 M6
N9 M2

Perfectly Centered

Note:  R3 is the angle of the letters, put R3+0.0 to make sure it stays at 0 Degrees of Rotation