<human>: One of my jobs is helping a doctor I know with his paperwork, faxes, invoices, emails, etc. Up until last week he had a separate office down the street from the place we both work (we're both technically self employed professionals) and he set two "shifts" where I'd come in and work and he'd ask for additional time as needed. 

As time went on we both got busier and often the "shifts" didn't work for me and/or he was so backed up with other work I'd often be sitting there idle on his dollar while he figured out what needed to be done.

Recently I recommended breaking up the hours and shifting to per diem. Now his private office and other work are combined in the same offices where I work so we see each other more and I'm able to adapt quicker to little tasks as needed.  I already shifted from billing say "10:30-2:30" and instead bill in 15 minute increments as needed, say 2.75 hours for 2 hours and 45 minutes. 

He seems ok with this but I'm not sure if that's the best way to do it. Obviously I want to be compensated for my work and he wants to get the best value for the work (I set my hourly rate for him before I agreed to work with him). I'm not trying to take advantage of his busy schedule but I'm also not trying to short change myself. He's very open and flexible about whatever at least it seems.

Am I doing it right? How do I bill for small 5 minute blasts of work, do I over estimate slightly or under estimate?

How do you run your billing in cases like this?

Can you succinctly summarize the text in 1-3 sentences?
<bot>: Doing per diem work for a doctor, sometimes tasks take a few hours other times it's many little tasks throughout the day, how should I bill increments of time in a way that's fair for both of us?