The proration percentage is based on Connecticut income earned while a resident of CT and CT sourced income divided by total income.
If all of your income was earned while a resident of CT or is CT sourced income (ex. from a CT employer or CT rental property), then there would be no proration for tax due.
See page 8 of the CT instructions for additional explanation: https://authoring.ct.gov/-/media/DRS/Forms/1-2018/Income/CT-1040NRPY-Online-Booklet_1218.pdf?la=en
