Step delay and pulse width for 28BYJ-48

I'm using the 28BYJ-48 stepper to drive a mini CNC machine, I use the arduino nano shield v4 and 3 a4988 and wiring 28BYJ-48 as a uni-polar.
I noticed that using the grblmain library making the motor to work improperly and skip steps.
I tries changing the pulse width to 20u but I'm not sure if its enough and what is the minimal pulse width and delay between steps. I couldn't even find the data in any datasheet.
so... can anyone tell what is the minimum delay between steps and minimum step width for the 28BYJ-48?

this is the current grblmain configuration

$0=64.000 (x, step/mm)
$1=64.000 (y, step/mm)
$2=1024.000 (z, step/mm)
$3=20 (step pulse, usec)
$4=250.000 (default feed, mm/min)
$5=250.000 (default seek, mm/min)
$6=24 (step port invert mask, int:00011000)
$7=25 (step idle delay, msec)
$8=10.000 (acceleration, mm/sec^2)
$9=0.050 (junction deviation, mm)
$10=0.100 (arc, mm/segment)
$11=25 (n-arc correction, int)
$12=3 (n-decimals, int)
$13=0 (report inches, bool)
$14=1 (auto start, bool)
$15=0 (invert step enable, bool)
$16=0 (hard limits, bool)
$17=0 (homing cycle, bool)
$18=0 (homing dir invert mask, int:00000000)
$19=25.000 (homing feed, mm/min)
$20=250.000 (homing seek, mm/min)
$21=100 (homing debounce, msec)
$22=1.000 (homing pull-off, mm)

now I'm Starting to wonder, if the a4988 is looking on a rising edge, is the pulse width even configurable...

The pulse width is only used for communication between the Arduino and the A4988. It does not need to be changed for different motors. The Allegro datasheet says that the A4988 is happy with a pulse with of 1 µsec.

AFAIK it is necessary to modify 28BYJ-48 before it can be controlled by an A4988 as the A4988 only has connections for a 4-wire bipolar motor.

The 28BYJ-48 gearbox introduces a lot of backlash which will make accurate positioning difficult, if not impossible.

Stepper Motor Basics
Simple Stepper Code

I did modified the 28BYJ-48 and wired it correctly with 4 wires.
I hope that the backlash will be insignificant because I really don’t need it for professional work, and the tool path should not be very complex.
In any case, if the machine I build will work OK I intend to upgrade the motors.