14 tpi is a funny pitch for a leadscrew, 0.125 or 0.1 inch would be more common. What sort of lathe is it?

Anyway 1 motor turn moves the carriage 1.25/14 = 5/56 inches. You want to calculate the steps per unit, or steps per inch. You need 56/5 turns to move 1 inch, so (800 steps/rev) x 56/5 = 8960 I think.