First you need to find the length of the expected course, and the desired start time of the first boat. The first boat to start will be the one with the highest PHRF. For the next slowest boat take the PHRF difference between him and the slowest boat and multiply that by the length of the course (in nautical miles). That’s how many seconds after the first boat the second one should start. Repeat for each subsequent boat.
It’s easiest to do with a spreadsheet. You can put boat names in column A, PHRF in column B, and the formula to do the calculation in column C.