Lighting up one by one

Lighting up one by one

#include "FastLED.h"
#include <SPI.h>
#define NumPixels 1500       // number of LEDs in strip - count starts at 0, not 1
#define Datapin 11           // datapin - green on apa102
#define Clockpin 13          // clockpin - blue on apa102
#define DataRate_Mhz 15
CRGB leds[NumPixels];

int g, r, b, z;
int head=-1, tail=(head-6);
float Speed;
float previousmicros;

void setup()
{
 FastLED.addLeds<APA102,Datapin,Clockpin,BGR,DATA_RATE_MHZ(DataRate_Mhz)>(leds,NumPixels);  // data rate is worth playing around with, if left untouched it goes mental
 Serial.begin(115200);                                                       // set the speed at which serial monitor will write at
 fill_solid(leds, NumPixels, CRGB(0,0,0));                                   // clear lights
 FastLED.show();                                                             // show the blank
 delay(3000);                                                                // delay so everything can be ready
}

void loop() 
{
 Speed = 8.47*1000000, g=255, r=0, b=255;               //speed is how long I would like for it to complete from LED 0 to the last LED
 while(head<=NumPixels-1)
 {
float currentmicros = micros();
float LEDInterval = (Speed/NumPixels); 
 if(currentmicros-previousmicros>= LEDInterval)
 {
  Serial.print("Head Equals "); 
  Serial.println(head);
  Serial.print("Interval Should be "); 
  Serial.println(LEDInterval);
  Serial.print("Interval Equals "); 
  Serial.println(currentmicros-previousmicros);
  Serial.print("Time Equals "); 
  Serial.println(micros()); 
  Serial.println("");
  previousmicros = currentmicros;
  leds[head] = CRGB(g,r,b);                 // head
  leds[tail] = CRGB(0,0,0);                 // tail
  FastLED.show();                           // show
  ++head;                                   // further the lights
  ++tail;
 }
 }
}