- Heroes in Heroes 3

If we are talking about Sandro's specialization, I think you are confusing the secondary skill "Magic" and the specialization "Magic." These are two different things. The secondary skill provides a +5/10/15% bonus, depending on its development level (basic/advanced/expert), to the base skill. The "Magic" specialization modifies the secondary skill, increasing it by +5% per level. That is, it adds +0.25% (5.0% - 5.25 - 5.5 - 5.75, etc.) to the base skill...

For example, Sandro has a basic level of the secondary skill, which gives +5% to the power of his offensive spells. For each level gained, Sandro receives +5% to the skill value,
i.e., 5% (basic) + 5%*1 (Level 1) = 5.25% (and not 10%, as you calculate)... At level 20, we have 5% + (5%*20) = 5% + 100% = 10%. (The increase is highlighted in red, i.e., +100% is a doubling of the value).

So, the general formula looks like this:

Damage bonus = (5/10/15%) * (1 + 0.05 * L), where L is the hero's level.
The maximum level is 108, so Sandro's maximum bonus is 15% * (1 + 0.05 * 108) = 96% to the power of offensive spells...