There is a way to make 1.5, 2.5, 3, 3.8 but it'll be more difficult (I'm not sure whether there is an easier way to make it)
For me, I'll prefer there's a constant increment. For example,
Level 1: x1.5 Strength
Level 2: x3.0 Strength
Level 3: x4.5 Strength
Level 4: x6.0 Strength
Choose the function "Arithmetic" and you'll see this
1.00 + 1.00
Click on the first 1.00 and choose again the "Arithmetic" function again which it should be like this
(1.00 + 1.00) + 1.00
Then click again on the first 1.00 and make it 1.5. Then click on the second 1.00 and choose "Conversion - Convert Real to Integer" and search for this function "Unit - Level of
Ability for Unit" Set the Ability to the ability that you want. Remember to adjust the Arithmetic Operator (+,-,x,/)
Click OK until you see the 1.00 (the third 1.00) which haven't manipulate. Click the 1.00 and again, manipulate it to the Strength of Unit.
It will look like this
-
Unit - Cause (Triggering unit) to damage (Target unit of ability being cast), dealing ((1.50 x (Real((Level of Ability for (Triggering unit))))) x (Real((Strength of (Triggering unit) (Include bonuses))))) damage of attack type Spells and damage type Normal
If you can't get it, I think you can get a sample from my spell,
Blessed Hammer 1,5b.
Open up the map and search for the BH_damage and click on it and see how I manipulate it.