Main destination of Schengen visa

The main destination rule:

• If you need visit one of the states for a specific, demonstrable reason, that is the main destination. Example: If you have a job interview in France and decide to add the weekend after in Italy on tourism, France would be the main destination.
• If you have no reason to single out one destination, the one where you spend the most days is the main destination. Example: Three days in France, six days in Italy, four days in Austria means Italy would be the main destination.
• If you spend the same number of days in different states, the first entry is responsible for the visa application. Example: Five days in Germany, five days in Poland, and Germany would evaluate the visa.

Your question's premise is incorrect. You write

the Schengen visa's main destination is the place where I must spend most of my days in Schengen countries

In fact, the main destination country, if one can be determined, is the country that must evaluate your visa application.

In Article 5, the Schengen Visa Code says:

Article 5

Member State competent for examining and deciding on an application

1. The Member State competent for examining and deciding on an application for a uniform visa shall be:

   (a) the Member State whose territory constitutes the sole destination of the visit(s);

   (b) if the visit includes more than one destination, the Member State whose territory constitutes the main destination of the visit(s) in terms of the length or purpose of stay; or

   (c) if no main destination can be determined, the Member State whose external border the applicant intends to cross in order to enter the territory of the Member States.

Thus, as noted in o.m.'s answer, the main destination may be determined in terms of either the length or the purpose of your visit to each country. As noted in dda's answer, when considering length, the threshold is not a majority of the days but rather a plurality, although as you can see even that threshold is not explicit in the code.

So if your trip to the Schengen area is for the purpose of attending a conference in France, you could argue that France is your main destination even if you will spend more time in Spain.

If your purpose in each country is the same, then your main destination in terms of the length of stay is Spain.