Expression Selectors and blend modes

Hi

The Range selectors have an Amount and a Mode (blend Mode) property. The Expression Selector does not.
I have an expression that is already working, but I want to add blend modes to it, so it can blend with previous selectors, just like two range selectors can. I have made a very to the bone example, so I can focus on the blend mode funtionality only. I have this expression:

var thisSelectorValue = 100; // (-100..100)
var amount = 50; // (0..100), like Range Selector's Amount

var thisMultiplier = amount/100*thisSelectorValue/100;
selectorValue*thisMultiplier;

The above makes sense. It multiplies the previous selection value for every character with .5. So if they where 100 % selected from the previous selector, they are now 50 % selected. This behavior will be the same as the "Intersect" blend mode.
I'm struggling making an "Add" blend mode. If I multiply the selectorValue with something it works, but if I use addition or subtraction it doesn't. This will do nothing:

var thisSelectorValue = 100; // (-100..100)
var amount = 50; // (0..100), like Range Selector's Amount

var thisMultiplier = amount/100*thisSelectorValue/100;
selectorValue+thisMultiplier;

All I did was to change the * with a +. I would expect characters that where 0 % selected to now be 50 % selected. But that doesn't happen. I also tried adding 50 instead of .5, but that didn't work either.

Does anybody have the clarity to shine some light on this for me?

Thanks,

Jakob