I used the sunny 16 rule ;P
B&W paper is about 10 ISO, so I guessed that color paper would also be in that area ISO. In sunny conditions, a good exposure would be 1/10th of a second @ f16. However, the proposed ideal f-stop for the dimensions of my shoebox are f237, which is roughly 8 stops from f16.
1/10th -> 5 -> 2 -> 1" -> 2" -> 4" -> 8" -> 16" -> 32"
On a sunny day, at f237 and ISO 10, the ideal exposure is roughly half a minute. But it was a cloudy day and a bit dark, so for the hallway I increased another 3 stops (32" -> 1' -> 2' -> 4') plus reciprocity failure just in case (4'+2'). And for the stairwell, I decreased another 1.5 stops because there was less outdoor light, so that was about 15 minutes.
I used this website to calculate the ideally focused pinhole size for my camera here -->
http://www.mrpinhole.com/calcpinh.php
Note: some of the calculate fields aren't linked and won't update themselves, so you have to enter data from the right to the left. It's kind of confusing at first, but it was easier than calculating it myself.
I recently exposed some negatives (not paper) and actually attempted to measure via light meter, but my dial doesn't go to f237 so I had to guess the equivalents again. At least it was a lot less math on my part. o_O