Added more information on lscale

README.md

@@ -359,7 +359,7 @@ 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).
+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.
 
 For example you can replicate FAST's delayed SFH using:
 ```