Time, negative time, multiplication by numbers

Hi. I have a table in which I want to calculate an adjusted amount based on time spent over a threshold. And it's giving me a headache with VALUE, SPILL and divide by zero errors. I think I read before you can't have negative time which could be casuing me issues but I also know you can substract one time from another as long as it doesn't end up with a negative.

So, this is what I have.

Cell B8 in 0% format, which is an adjustment rate.
Cell B9 in HH:MM format, which is a time threshold.

Column A "Billable time" in HH:MM format.
Column B "Billable amount" in 0.00 format.
Column C "Fixed price" in 0.00 format.
Column D "Charge" in 0.00 format.

The above values are simple values with no formulas.

In Column E "Adjusted charge", I would like to calculate a value in 0.00 format based on the above values.

If "Fixed price" has a value, then the result should be "Fixed price". Everything else can be ignored.
If "Fixed price" has no value or zero, then...
If "Billable time" is less than the time threshold in B9, then the result should be the "Charge".
If "Billable time" is above the threshold in B9, then the threshold should be subtracted from it, then the remainder adjusted by the adjustment rate in B8.
Finally the adjusted remainder, i.e. the time above the threshold, should be added back to the threshold.

It's basically calculating, or should be, a reduced rate if time spent is over a certain amount but only for that time over. The time before the threshold is charged at the nromal rate.

I've tried splitting out the formulas into separate columns but I'm hitting brick walls each time. Help would be hugely appreciated.

Thanks.