There is a function which returns level of ability being cast (it's an integer number).
You just calculate the amount via a formula.
For example if your Holy Light heals 100/150/200, then the formula is basically 50 + ( [level of ability being cast] * 50)
Level 1: 50 + (1*50) = 100
Level 2: 50 + (2*50) = 150
Level 3: 50 + (3*50) = 200.
In many cases, you can find the formula easily. It may be harder for some - e.g. Holy Light heals 100/150/250.
Of course if the spell heals like this: 100/139/232.25, then you may have pretty hard time figuring out the formula, in which case it may simply be batter to save each amount of heal in an array and set the values upon map initialization.
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Health of a unit is a real number (number with decimal point - e.g. '10.850'), however the function for finding level of ability returns an integer number (number without decimal point - e.g. '5').
So when you are setting up how much you want to heal unit (when you're setting the real number), also choose the function called 'Conversion - Convert integer into real' which does exactly what you want - it converts an integer number into real number.
The function for finding out level of ability is in list of functions for integer number and is called something like 'Unit - Level of ability for unit'.