Starting with 300 base magicka, plus 50 from Highborn and 50 from Savos Aren's Amulet, results in 400 magicka and 12 points regenerated per second.
Use a mix of regenerate and fortify magicka effects for optimal results. If that same character gains an extra 100 magicka instead, the regeneration rate will be 12 points per second (400 * 3%). For example, a character with 300 base magicka and 100% magicka regen bonus will regenerate 18 points per second (300 * 6%). However, magicka regeneration items will be preferable if the Dragonborn has already been investing a lot in base magicka as they level up. While the magicka bar will appear to refill more slowly, this is simply because there is more total magicka to regenerate. If that same character gains an extra 100 magicka instead, they will regenerate their 200 magicka at 3% per second, or 6 points. For instance, if a character with 100 magicka gains an extra 100% magicka regeneration, that character will regenerate their 100 magicka at 6% per second, or 6 points. This means that an increase to magicka is usually preferable to an increase to magicka regeneration, as the former naturally comes with a certain amount of the latter. Thus, having 100 maximum magicka yields 3 magicka per second. Magicka Regeneration naturally regenerates at 3% of the Dragonborn's maximum magicka per second and at 1% in combat.