This is how I think I'm zeroing beatsin16(..) value by passing it a timebase of GET_MILLIS() and a phase shift of 49151

FastLED Beatsin16 Testing

//I have a test matrix setup with 5 x rows of 10 ws2811 LED's all wired left to right.
//I needed a way to zero beatsin16 for sinelon type effects and found through testing that a phase shift of 49151 does the job.


#include "FastLED.h"

#define NUM_LEDS 50
#define POWER 50 //power I want to limit my leds to. 

// Data pin that led data will be written out over
#define DATA_PIN 14


CRGB leds[NUM_LEDS];

void setup() {
 FastLED.addLeds<WS2811, DATA_PIN, RGB>(leds, NUM_LEDS);
 FastLED.setMaxPowerInMilliWatts(POWER);
 Serial.begin(115200);
 Serial.println("This nano has a burnt out gpio pin D11");
}


uint32_t timebase=0;
uint32_t printTimeInterval=20; //time between serial plotter prints.)
uint32_t printTime=printTimeInterval;  

uint32_t phase=49151; //phase shift required to bring back to position zero when compined with timebase of GET_MILLIS(). Found by trial and error. I don't knwo what I'm doing.

void loop() {

fadeToBlackBy( leds, NUM_LEDS, 255);
 EVERY_N_SECONDS(10)
 {
    timebase=GET_MILLIS(); //every 5 seconds I'm resetting timebase so that all five rows of leds will start at 0 position
 }
 
   uint16_t pos = beatsin16(25,0,65535,0,0); 
   uint16_t posWithTimebase= beatsin16(28,0,65535,timebase,phase);
   uint16_t row1Pos = beatsin16(10,0,9,timebase,phase); 
   uint16_t row2Pos = beatsin16(15,0,9,timebase,phase)+10;   //offset for each row position (10 leds per row)
   uint16_t row3Pos = beatsin16(20,0,9,timebase,phase)+20; 
   uint16_t row4Pos = beatsin16(25,0,9,timebase,phase)+30; 
   uint16_t row5Pos = beatsin16(30,0,9,timebase,phase)+40; 

   leds[row1Pos]=CRGB::Blue;
   leds[row2Pos]=CRGB::Red;
   leds[row3Pos]=CRGB::Green;
   leds[row4Pos]=CRGB::Yellow;
   leds[row5Pos]=CRGB::Purple;
   
   
  if(millis()>printTime)
  {
    printTime=millis()+printTimeInterval;
    Serial.print(row1Pos);
    Serial.print(", ");
    Serial.print(row2Pos);
    Serial.print(", ");
    Serial.print(row3Pos);
    Serial.print(", ");
    Serial.print(row4Pos);
    Serial.print(", ");
    Serial.println(row5Pos);
  }
   
   FastLED.show();
}