edit because testing shows you have to go to 255 to get all the way to the end and shortening the delay vs using the map function may be cheaper

You are calling the wrong method to get the count of pixels.
Pixels() returns a pointer to the bytes of raw data.
PixelCount() returns the number of pixels.

You can also just map the 0-100 to 0-255, by using i * 255 / 99

also, you can set all the pixels to a color by calling ClearTo()

Ha! I guess that's a testament to your code that it's working very well with my stupid in it. Even dumber is that I have it stored in a static variable in my Chamber object. But...

I was starting with the count method as I'm planning to have a 6x24 matix that I will need to light in zones. in playing with topologies I foind it difficult to define a square from 2,4 to 4,6 and then turn it white. my plan was to eventually turn my method into a "do it the hard way" generic "change color of random pix" by feeding it maps from config. I'm sure you have a way to do this, I just haven't looked hard enough at it as I've been fighting with simple things and kicked the can down the road for now. That said I see that ClearTo() has a start and stop so if I can get the points in each matrix segment then this still works for my generic method.

Thank you for your help with this. Let me make a couple changes and get back to you. As of now, my original quesion has been answered with your original pointer to use the measured progress method.

To complete the answer with corrected code, here are the resulting functions in my class that accomplish stated goals using @Makuna suggestions. everythign works and the results in smaller than my original code.

notes:

• _fadeSpeed is set in class to allow adusting the rate of fade (set to 254 when no fade, set to 1 when slow fade).
• the initial setLevel(int oLvl) allows for reading from analog read to set ambient levels (used with door switches and ntp server to adjust levels)
• _setLevel(RgbColor) is called by a few other class routines (for now) and still needs extended to manage my "bright box". but this is much simpler for now.

void Chamber::setLevel(int oLvl){
        // map the analog signal to the limits we set for non-alert brightnesss for this chamber
        // send that as a white level to setLevel for standard
	setLevel( RgbColor( map(oLvl, 0, 1024, _minNormalBright, _maxNormalBright) ) );
}

void Chamber::setLevel(RgbColor oLvl){
        // run through 255 possible itterations of color blend
        // use _fadeSpeed to jump itterations
	for ( uint8_t q = 0; q <= 254; q += _fadeSpeed ){
		_setLevel(RgbColor::LinearBlend( _currRgbColor, oLvl, q));
	}
       // when going to black fast we tend to not get to the last step to oLvl once to force it
	_setLevel(oLvl);

        // store oLvl as current string color
	if( !_alert ) { _currRgbColor = oLvl; }
}
void Chamber::_setLevel(RgbColor color){
	// we don't need a for loop to hit each pix.. who knew?
        // that would be folks to read ALL the API docs. 
        _strip->ClearTo(color);
	_strip->Show();
}