Adam,
I'm running 4.2 now and have set a Project's "Kind" as "Daily Fixed Amount" and the "Daily Amount" as (say) $500. However, if I add two Task entries against this project, one for four hours in the morning, and another for four hours in the afternoon of the same day, the Total Amount for the day is 2 x $500. (Even though there is a daily rate, I still need to account for the actual hours and various tasks.) This happened whether the Tasks' Rate was blank (i.e. left as "optional" - so it uses the project rate) or if I specified some value in it.
Am I missing something? For a Daily Fixed amount, I should be able to run up any number of hours in that day (up to 24) and still only tally to that fixed amount.