This might help you.
I made this for you. It's not perfect, but it works. What I made was just to create a unit to calculate the damage. So if you crit, the blink will have the crit dmg. If you got bonus dmg it will be there to.
Checklist.
-Magic immune to prevent others to dmg the dummy.
-Caster Invunerable while casting so that he can't be stunned et.c.
-500000hp of dummy, even if your hero reaches this amount of dmg, we can call this the cap of blink dmg.
-Added some effects to try to make it look good.
What you can change to fit your map.
-The cap dmg of the blink. Do this by changing the hp of the wisp dummy. Maxhp = cap dmg.
-Change the effects, just added those to try to make it look good.
-Stats of the blink such as range, you know the rest.
Hopefully it helped at least a little bit ...