Yes I think you should use interpolation. Are you doing a credit review of a bank/financial institution by the way?
If your trade receivables do not contain significant financing component, you can recognize lifetime expected credit losses right on initial recognition.
Moreover, as a simplification, you can use so-called provision matrix.
Check out:
*****://***.pwc.***/ca/en/accounting-advisory-services/publications/us2014-06-ifrs-9-expected-credit-losses.pdf
http://***.ey.***/Publication/vwLUAssets/Applying_IFRS:_Impairment_of_financial_instruments_under_IFRS_9/$FILE/Apply-FI-Dec2014.pdf
http://***.ifrsbox.***/ifrs-9-expected-credit-loss/