The ratio of infrared to visible light is variable so the E.I. you set on your visible-light meter can vary too.
I haven't shot Efke IR820 behind a IR720 filter in the 35mm format but have exposed maybe a hundred rolls of 120 and maybe a hundred sheets of 4x5 and 8x10. Connecting what I saw, what I did, and what I got, the E.I. values that work for me are:
Front-lit sunny day subjects: E.I. = 1.5
Back-lit sunny day subjects: E.I. = 1.0
Open shade: E.I. = 1.0
Overcast, stormy, dramatic: E.I. = 0.5 maybe 0.25
The good thing about infrared is that there is no blame if you didn't get what you expected. You can't previsualise what you can't see. Something weird/wonderful usually turns up.