Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- There is nothing wrong with calculating zakah according to the solar calendar.
2- Adjustment of the rate becomes due in order to make up for the 11 days difference.
In his response to your question, Prof. Dr. Monzer Kahf, Professor of Islamic Finance and Economics at Qatar Faculty of Islamic Studies, states:
Zakah may be calculated according to the solar calendar and then adjustment of the rate becomes due in order to make up for the 11 days difference.
The Zakah rate for the solar calendar basis becomes 0.025 + 0,025 x 11/354 = 2.578.
This is not only logical but also adopted as a Fatwa by the International Shariah Board of Zakah of the Kuwait Zakah House.
Almighty Allah knows best.