The minimum annual taxable salary level should be met for the duration of the 30% ruling. For 2020 the minimum salary level is EUR 38.347 (after 30% ruling calculation). This amount will change each year and every year the taxable salary should be at least this amount. A reduced salary level (for 2020: 29.149 after 30% ruling calculation) could be applicable when you are younger than 30 and hold a master degree. As soon as you become 30, the normal salary level needs to be met for the further duration of the 30% ruling.
With regards to the reduced salary level, if the master diploma have been issued outside the Netherlands then the Dutch tax authorities could request for an additional evaluation of the diploma to make sure the degree is equal (or higher) to a master degree in the Netherlands. The evaluation needs to be done by NUFFIC. For more information I refer to the following website: https://idw.nl/en/credential-evaluation.html.