added fixed-rate and hourly-rate for customer, project and activity (#304)

var/docs/timesheet.md

@@ -20,14 +20,32 @@ Please note: if time rounding is activated (which is the default behaviour), the
 
 ## Rate calculation
 
-The rate of a timesheet entry can be calculated from several settings.
+There are two rate types:
 
-It can have two values dedicated to the entry itself:
-- if a fixed rate is set, the rate of a record is set to this exact value no matter how long the duration is
-- if a hourly rate is set, it will be used to calculate the record rate by using it multiplied with the records duration
-- each of the above can be set to 0, to set the records rate to 0
+- __Fixed rate__: the value will be used to set the records rate, no matter how long the duration is
+- __Hourly rate__: will be used to calculate the records rate by multiplying it with the duration (see below)
 
-If none of the above was set the users rate will be used to calculate the records rate.
+If any of the above is set to 0, the records rate will be set to 0.
+
+While calculating the rate of a timesheet entry, the first setting that is found will be used (in order of appearance):
+
+- Timesheet fixed rate
+- Activity fixed rate
+- Project fixed rate
+- Customer fixed rate
+- Timesheet hourly rate
+- Activity hourly rate
+- Project hourly rate
+- Customer hourly rate
+- Users hourly rate
+
+If neither a fixed nor a hourly rate can be found, the users rate will be used to calculate the records rate.
+In that case and the users rate is not set or equals 0, the records rate will be set to 0.
+
+The calculation is based on the following formular:
+
+- __Fixed rate__: `$fixedRate`
+- __Hourly rate__: `$hourlyRate * ($durationInSeconds / 3600) * $factor`
 
 Please see also the configuration chapter about [hourly rates for timesheet records](configurations.md) to see how you 
 can apply configurable multiplying factors based on day and time.