Post above is true to some extent, you need to make your own quadratic light values and choose which fits your need rest is finding the distance light will be invisible which will be your already defined radius meaning

For a start pick: constant 0.85

Linear 1.0

Quadratic 0.06

//If we want attenuation to match light radius we first need to define at which attenuation light becomes invisible
//lets assume at 0.004 (rounded 1.0 / 256.0)
float Attenuation(float dstx)
{
return 1.0 / (LConst + LLinear * dstx + LQuadratic * (dstx * dstx));
}

float CalcLightAttenuation(float REALradius)
{
float fdst = n3ddistance(LPos, vertex_pos);
float kompot = fdst / LRadius;
//float intensity = clampme(1.0 - kompot);
return Attenuation(REALradius * kompot);
}

now call CalcLightAttenuation(your radius);
FinalColor 

tcol is rgba value of the texture



vec3 ambient = lightColor * MAmb * (intensity*attenuation);

	vec3 diffuse = lightColor * MDiff * (intensity*attenuation);
	float lspec = Specular();
	vec3 specular = lightColor * (lspec * MSpec);



		color = vec4((ambient+diffuse+specular)*tcol.rgb, tcol.a);

I am sorry i didnt provide full code now it has everything you need LRadius ia the light radius

uniform vec3 LPos;
uniform vec3 LDiff;
uniform vec3 LAmb;
uniform vec3 LSpec;
uniform float LRadius;
uniform float LConst;
uniform float LLinear;
uniform float LQuadratic;
uniform int DirectionalLight;
uniform vec3 LDirection;
uniform float z_far;

uniform vec3 MAmb;
uniform vec3 MDiff;
uniform vec3 MSpec;
uniform float MShininess;


//If we want attenuation to match light radius we first need to define at which attenuation light becomes invisible
//lets assume at 0.004 (rounded 1.0 / 256.0)
float Attenuation(float dstx)
{
return 1.0 / (LConst + LLinear * dstx + LQuadratic * (dstx * dstx));
}


float CalcLightAttenuation(float REALradius)
{
	float fdst = n3ddistance(LPos, vertex_pos);
	float kompot = fdst / LRadius;
	//float intensity = clampme(1.0 - kompot);
	return Attenuation(REALradius * kompot);
}





	vec3 light_vert = normalize( vectorAB( LPos, vertex_pos ) );
	float intensity = max(-dot(light_vert, normal), 0.0);


	vec3 lightColor = LAmb + LDiff;
	float lightMax  = max(max(lightColor.x, lightColor.y), lightColor.z);
	float REALradius = (-LLinear +  sqrt(LLinear * LLinear - 4.0 * LQuadratic * (LConst - (256.0 / 5.0) * lightMax))) / (2.0 * LQuadratic);
	float attenuation = CalcLightAttenuation(REALradius);

vec4 tcol = texture2D(face_tex, texcoord);

	vec3 ambient = lightColor * MAmb * (intensity*attenuation);

	vec3 diffuse = lightColor * MDiff * (intensity*attenuation);
	float lspec = Specular();
	vec3 specular = lightColor * (lspec * MSpec);

Final_color = vec4((ambient+diffuse+specular)*tcol.rgb, tcol.a);