Doing arithmetic in GUI is somewhat inconvenient, but not too bad for a simple formula like this. Here's how to do it:
1) In the Action, select Damage Unit
2) Set the unit to damage to "Target unit of ability being cast"
3) Click the damage value to edit it (initially set to 500)
4) Click the drop down window and find "Arithmetic"
5) Now, you get two values: initially set to "1.00 + 1.00"
6) Select the first "1.00" and edit it to your base ability damage (in your case, 50)
7) Select your second "1.00". This will be our ability scaling value.
8) Inside here, click the drop-down menu. Select arithmetic
again. It will autofill "1.00 + 1.00" again.
9) Select the first "1.00". Now since we need to use Level of Ability (which is an integer) but this is a real, we need to convert.
10) Find "Conversion - Convert Integer to Real". It will autofill "Point value of Footman". Click this to edit it.
11) Scroll down to "Unit - Level of Ability For Unit". Select the red text that says "Ability" inside this.
12) Find your custom ability and select it.
13) Select OK twice to get back to Arithmetic. Now, the screen should say "Real(Level of ___ For Unit) + 1.00"
14) Select the "+" operator, and change it to "x" inside the drop down box (multiplication)
15) Select the second "1.00". Change this to "100.00", because we want to multiply the level by 100.
16) It should now read: "50.00 + (Real(Level of ___ For Unit) x 100.00)
17) Press OK as many times as needed.
Here's a short video of me doing it. Slightly cut off, but it shows the important parts:
Remember: Order of Operations is
IMPORTANT! Parenthesis always go first. 50 + (Real(Level of ___ For Unit) x 100.00) is NOT equal to (50 + Real(Level of ___ For Unit)) * 100