Well if 18,000 works then there is something wrong in the figures you supplied, motor 200 steps per turn x 20 turns of screw to move 1 inch = 4,000, now figure in the microstepping and for 18,000 that would be 1/4.5 microstepping, you don't get that
Anyway if 18,000 is working then stick with it for now, next thing to do is go back to the motor tuning and start messing with the Velocity and acceleration settings, best to lower the accel near the start and increase your velocity a step at a time then use your keyboards up/down arrow keys to jog back and forth. When your motors start squealing (missing steps) turn the velocity down a bit until they are again OK. Next start increasing the Accel and repeat. Find the sweet spot for your setup for Vel and Accel and yu are done, save and move to the next axis.
Hood