Regarding those who say it is okay, Imam as Sanaani mentioned “A clear response against them….” is the following narration
ابْنَ عُمَرَ ـ رضى الله عنهما ـ عَنِ النَّبِيِّ صلى الله عليه وسلم أَنَّهُ قَالَ “ إِنَّا أُمَّةٌ أُمِّيَّةٌ، لاَ نَكْتُبُ وَلاَ نَحْسُبُ الشَّهْرُ هَكَذَا وَهَكَذَا ”. يَعْنِي مَرَّةً تِسْعَةً وَعِشْرِينَ، وَمَرَّةً ثَلاَثِينَ
Ibn `Umar (may Allah be pleased with him) narrated: The Prophet (ﷺ) said, “We are an illiterate nation; we neither write, nor know astronomical calculations/computations. The month is like this and this, i.e. sometimes of 29 days and sometimes of thirty days.” [Bukhari and Muslim]
Imam Sideeq Hassan Khan said that calculations in figuring out the days and months due to the stages of the moon is an innovation by the consensus of the [Muslim] nation.
The Lajnah Daimah said:
“Returning to astronomy in affirming the lunar months for the starting of acts of worship or exiting from it without using eye sight is an innovation which has no good in it nor does it have any evidence for it in the legislation.” End translation.
And Ibn Daqeeq Al Eid said: “That which I say is that calculations is not permissible to be relied upon in regards to fasting” End translation.
[Benefits taken from Itihaaful Anaam of Shaykh Muhammad Hizaam, page 11-12]
Majid Jawed Al-Afghanee