It was based on guestimate, and the exposure depends greatly on the type of light; so with direct midday sun you may well get an EI of 3 (my old Weston Meter doesn't deal in ISO, so I assume a Weston value of around 1.5, which would be slightly slower than EI3, maybe EI2). However, with tungsten light it's much more orange (hence the requirement of a blue filter for colour film).
My thinking went along the lines of... if dark amber would be a safelight for paper, then an orange light is going to slow things down with reference to sunlight. I did try a first exposure of 1 minute 45 seconds, but it obviously wasn't enough, so I just added some more and made it a round 3 minutes!
Nothing scientific I'm afraid! Just best guesses. The great thing about working with paper is that I can take a shot, whizz into the darkroom, develop the paper and get an idea of anything needs changing with the exposure; if the exposure is out, I don't even bother to fix (just stop developing in a stop bath of tap water).
I'm going to retake this shot with a different background as I hate this one!
Hope it helps?!
PS: Here's another paper negative image of an Allium plant seed head...