Clarified lscale further

README.md

@@ -359,7 +359,9 @@ For this reason in FAST++ you have the option to build the template library on t
 
 Using custom SFHs is easy. First you have write down the analytical formula in the ```CUSTOM_SFH``` parameter. This formula should return the star formation rate (in arbitrary units) as a function of time since onset of star formation ```t``` (given in years), where ```t=0``` is the birth of the galaxy. The formula can involve any math function, including: ```abs```, ```acos```, ```asin```, ```atan```, ```atan2```, ```ceil```, ```cos```, ```cosh```, ```exp```, ```floor```, ```log``` (natural logarithm), ```log10``` (base-10 logarithm), ```pow```, ```sin```, ```sinh```, ```sqrt```, ```tan```, ```tanh```, ```min```, ```max```, ```step``` (returns ```1``` if argument is zero or positive, and ```0``` otherwise). See the documentation of [tinyexpr](https://github.com/codeplea/tinyexpr) for more detail.
 
-In addition, if your SFH is parametrized by one or more parameters, the formula can reference these parameters by name. For this to work, your parameters must be listed in the ```CUSTOM_PARAMS``` parameter as an array of strings, and for each parameter ```x``` you must provide the grid parameters as ```X_MIN```, ```X_MAX``` and ```X_STEP``` (with ```X``` in upper case by convention, but lower case works too). Implicitly, FAST++ will always introduce the free parameter `lscale`, which is the normalisation factor of the SFH. This parameter is automatically determined using a linear fit, and is not part of the grid. It cannot be referred to within the SFH expression.
+In addition, if your SFH is parametrized by one or more parameters, the formula can reference these parameters by name. For this to work, your parameters must be listed in the ```CUSTOM_PARAMS``` parameter as an array of strings, and for each parameter ```x``` you must provide the grid parameters as ```X_MIN```, ```X_MAX``` and ```X_STEP``` (with ```X``` in upper case by convention, but lower case works too).
+
+NB: Implicitly, FAST++ will always introduce the free parameter `lscale`, which is the normalisation factor of the SFH (it is multiplied to the SFH value obtained from the expression). This is a special parameter that is automatically determined using a linear fit; it is not part of the grid, and it cannot be referred to within the SFH expression. But it is recorded as an output like any other SFH parameter.
 
 For example you can replicate FAST's delayed SFH using:
 ```