(newbie) ProjectTracking FTE calculation

I've looked around for what should be a simple answer. (BTW - what a great site this is, wow!)

- I am a project manager working in small/mid sized ERP implementation (not relevant, but throwing that out there)

- Using Excel to track projects, Projects is to expensive for what I do

- OK, FTE is Full Time Equivalent. If a Task takes 1 day to do and Sally is only working 4 hours a day on the project, the FTE of this task should be 2 days. It'll take Sally two days to do a one day task.

- What I know is this:
- Tasks all take X days to accomplish, this is provided by the slavedriver...errrr... project manager (me)
- Sally has a real job and can only devote 50% of her time to the project. Take Bob, he is only willing 10%. It'll take him (????) days to complete the 1 day task.

- So, this is simple. number of days of the Task * 8
- % the person is available to work on the project (let's call it "availability"
- divide the number of hours by "availability".
- round up to the next number


MY QUESTION:

Do any of the Project Managers out there have a better calculation than the idiotic one I have? I'm missing something....


Mucho Thanks,

That new guy...