I suggest that you keep your time entries in standard Excel format, which as you note has time as the decimal part of the number.
If you use proper Excel times, you can calculate differences with simple subtraction.
Then as a final step convert the time difference into hours and fractions of an hour.
You don't say whether there is any date information stored with your times. This makes a difference when calculating the time difference when start is before midnight and end is after midnight.
If the date is included e.g. 20/July/2010 23:00 to 21/July/2010 01:00 the difference will calculate to 2 hours.
If there is no date information, Excel shows ######## because it will not display negative times.
To overcome this and assuming that you are entering times with no date information, put start time in A1 (23:00) and end time in B1 (01:00)
then in cell C1 enter this formula:
Format the cell as h:mm
Cell C1 will display 2:00
Now convert the time difference to hours and fractions of an hour.
Format cell C1 as a number with say 4 decimal places
Enter this formula in cell C1:
Using your example with 06:30 in A1 and 14:00 in B1
C1 shows 7.5000
A1=06:30, B1 = 06:50
C1 shows 0.3333 i.e., a third of an hour.
Hope this helps