Just do a google search for "diablo ii skill calculator". From there I'd recommend chippydips or indiablo. Either will work.

They don't make it real easy though. You'll still have to do the math yourself. But you'll need them to get you the base damage for your level 40 (or whatever level it is) lightning, chainlightning, etc. Don't put all your synergies into the skill calculator, just put 20 points into the skill you want to check out, and list your total number of + lightning, nothing else.

One you have the base spell damage you'll have to count up your synergies for that spell (i.e. 20 from chain lighting, 20 from charged bolt, 20 from nova assuming you have it maxed = 60 synergy points). Then multiply by the synergy mod % (8% for lightning so 60 * 8% = 480%).

Next you need to get your mastery mod%. This is the total amount of lightning mastery (just look it up on your skill tab) and all your + lightning skill damge (eschuta, ormus, and facets). You need to do this seperatly because the skill calculators won's allow you to add in % from gear.

At this point you just multiply them all together. Total damage = base damage * (1+synergy mod) * (1+mastery mod).

Then you need to apply the -enemy resistance. If you have -100 enenmy resists you can multiply by 2; -50% would be like multiplying by 1.5, etc.

If you get really lost look for the thead I did about carrying infinity or not - I believe I stepped though the calculations there.