What is the formula for magic damage?
I have health & mana in Excel working from another post,
but could not find the magic calculations.
My character is L80 with:
Str 15
Agil 10
End 302
Intel 108

and firebolt is 1557
shadow light 1816
ice shards 665
with no other bonus/skill/equipment.

INT raises your mana pool. I may be incorrect, but I thought it also improves your chances of hitting a creature without resistance and also improves your chance to critical hit.

That means the formula is the average of the range of damage of the spell, plus all appropriate bonuses.

If a bolt spell does 200-800 damage, on average it will do 500, plus the corresponding bonus damage shown in your character sheet for that element.

My level 51 Pyromancer has a 320 fire bonus (from +fire damage equipment) and uses Pyromania for a boost to over 500.

Sounds about right, but I believe a higher INT also increases your base spell damage. We won't be getting the actual in game formulas though.

Exactly!! If you look through past posts a few people worked alot of equations for spell damage. A few things to keep in mind though is that the equations worked out was using data from multiple builds ago and things have change drastically. Also the crits have just been added recently so they are not included in the past equations. I am pretty sure Int does have an affect on these crits but it is unlikely that we will ever see the equation for this.