I'm DIY CNC Lathe use BOB Mach3 Parrarel port

I connect Proximity and config to index mach3

I test run put parameter spindle 300 rpm, and Strue show about 303 - 307 rpm

I try use wizards threading in mach3 turn, and export gcode this below

G0 G40 G18 G80 G50 G90
G00 X11
G00 Z0
G00 X10
M03 S300
G76 X8.5 Z-18 Q0 P1.5 J0.3 L0 H0.1 I0 C1 B0.025 T0

Spindle 300 RPM
Thread 1.5

but when result part thread is about 2.0

I try run code again I see in monitor I found number of mm per REV = 2.06

how to fix it's for same gcode P1.5
Sorry i'm newbie.