Belgium Passport holder, 2 prior METVs

Currently in Thailand until 19/10/2019 (returning to Belgium)

Applied for language school 3 weeks ago, received the acceptance letter yesterday. Since this letter is only valid for 1 month, the school has put starting date of studies from 19/10/2019 to 19/01/2020

I will apply for ED visa as soon as I am back in Belgium.

However I only plan to return to Thailand on 16/01/2020. This means there are only 3 days of studying remaining when I land. If I undestand correctly, ED visas are valid for 3 months at a time.

Is the ED visa start date as per acceptance letter or as per flight date? Will I have only 3 days remaining on my ED visa when I land or will I get a full 90 days?