PIC Timer and decimals

I am using a PIC 19F690 and I am trying to setup TIMER0 for my first time.
For the record I am using MicroCode and PICKit2.

Basically what I want to do is log a time period elapsed and calculate a litre(s) of liquid used during the elapsed time.

I have a sensor that I can accuratly calculate the litres/min flow rate of water going through a pipe.

So once I determine my flow rate I then want to log the elapsed time and multiply it by the flow rate to determine to water used.

The problem I have is this;

Elapsed time = 3 Seconds
Flow Rate = 10 Litres per second
Water used = (3x10)/60 = 0.5 litres

The problem I have is I do not know how to use decimal places and I dont think the above calculation will work, will it?

This is my untested section of code (Sec is to be calculated from the interupt routine, yet to be written);

Main:
Time = Sec ' Set as Start time
IF Flow = Lmin THEN ' If the water flow rate hasnt changed_
GOSUB GETFLOW ' keep checking until it does
ENDIF
Elapsed = Sec - Time ' Capture the time period Note: Sec updated by interupt
Wused = Elapsed * Flow ' Calculate water used during elapsed time
Flow = Lmin ' Set new Flow value for next period Note: Lmin comes from GETFLOW Routine
Tused = Tused + Wused ' Add water used during period to total used so far
GOTO Main:

I also need some help to setup the interupt routine to update SEC every 1 second.

If anyone can help me with this I would greatly appreciate it...




MOD Edit: Spam link deleted