I think that spreadsheet is still ok for calculating character sheet values.
Damage calculation is not always straightforward as it isn't clear where certain buffs are multiplicative or additive with one another or with CP.
I'm not too sure how much detail you want but if you use skill coefficients (for example from this website http://esoitem.uesp.net/viewSkills.php) and you know all the damage related equations you will come quite close to ingame damage values. For example, my calculated damage tends to be close to what is observed ingame