How do you find, for a specific flight, that flight's on-time % for a month of those flights?

ex: How can I find what is the average on time % of:

Date: January 1 - February 28, 2019

United 255 Austin to IAD (Washington Dulles)
and
Austrian Air 94 IAD to VIE (Vienna)

Is there an app or website for that?

I tried to do this on flightstats.com, and registered for the Pro account, but it only let's me search flight data for 1 historical day at a time
So I'd have to search 30 times, enter everything into a spreadsheet, average, etc.
yuck

There is a bureau of transportation website for the domestic leg, but no data on the intl part.