SWIY should get a scale, it's probably the best investment he can make.
That being said, if you know the total weight of what you have, you can divide it into easy to see fractions until it's small enough (halves, fourths, whatever). This will be very inaccurate but if SWIY insists on doing it without a scale he should at the very least do that.
To address the second part, I only took Pharmahuasca once (in a dream, not in real life), and I started with 40mg and just over an hour later I added 20mg, and from what I could tell it worked out very well. I may be wrong, but I think this is very common. I could be wrong about this, but I don't think it's really advisable if you're much more than 2 hours into the experience, since Erowid lists Syrian Rue's MAOI effect to last for 3-5 hours, so much more than 2 hours in it would probably just be a waste.
But perhaps someone more experienced should comment.
Edit: Listen to the guy below me. His advice seems much better.