It is 25% of the income received from the state teacher’s retirement system.
From Connecticut:
For taxable years beginning on or after January 1, 2018, in determining a taxpayer’s Connecticut AGI, a taxpayer is allowed a subtraction modification of 25% of the income received from the state teacher’s retirement system.
This modification applies to the extent such income is properly included in the taxpayer’s federal AGI for the taxable year.
Full Connecticut instructions (page 2):
https://portal.ct.gov/-/media/DRS/Forms/1-2018/Income/CT-1040-Online-Booklet_1218.pdf?la=en
