Overload manually overtime work

Question

Please, could you help me to better understand the calculating methods of MS Project because I don't well understand MS Project behaviour: To explain my problem, here is my context :
CONTEX
Calendar : Std (08:00-12:00 / 13:00-17:00) assigned to all tasks
Auto schedule activated
Resource R1 :
Calendar standard excepted for February 10 2014 with working time 08:00 - 12:00
Capacity ___
For February 10, 2014 = 100%
For February 11 to 15, 2014 = 50%
Cost rate___
for February 10 , 2014 Std rate 10€ and overtime rate 20€
for February 11, 2014 std rate 20€ overtime rate 40€
Task 1 :
Constraint Start not earlier than February 10, 2014 08:00
finish date February 11,2014 17:00
fixed Work = 15h
For the Gantt Chart, I display Resource usage view => 3 panels + 1 chart
On the left detail part, I enter 7h on the overtime work field (after manual insert of this field)
On the  right part of the resource usage view, I enter for :
February 10 , work 8 => overallocation automatically calculated  = 4 => Ok
February 11, work 7 =>  overallocation automatically calculated  = 3 => Ok
PROBLEM
But MS project calculates also overtime work, and I don't understand how because
For February 10 , overtime work  = 3,28h
For February 11, ovt work = 3,72
Firstly, no way to change this repartition, automatically calculated. Impossible to type data on ovt work field
Secondly, it's better to overload the ovt work of the Feb. 10 because this ovt rate is lower than the ovt rate of the Feb.11
Finally the total calculated cost  by Project = €327,19 but according to my own calculation it should be egal to €334,40
Feb 10 = 4h std time x 10 std rate  + 3,28h ovt x 20 ovt rate = €105,60
Feb 11 = (4 x 20 ) + (3,72 x 40) = €228,80
Total cost = €334,40
Thanks a lot for your help

Answer 1

No, you cannot manually enter Overtime work in the timescaled view of Task Usage or Resource Usage. Project will also not modify the amount of overtime based upon what is the most cost effective.  In my scenario, the overtime was split evenly based
upon the assignment units of 100% for both days.
I am seeing what I consider an error in the assignment values that Project is using.  When I did your scenario test yesterday (Feb 10) the resource was assigned 4 hours of work but at 100% assignment value.  The 4 hours from the edited calendar
but 100% because that was his/her max units.  The rest of the assignment was also at 100%, causing an overallocation.
However, today when I repeat the same scenario on a new file, I see the assignment units are incorrectly applied on Feb 10 - they are applied at 50% and the duration of the task blows up to 3.75 days.  It appears as though Project is assigning the resource
at what ever their max units are as of the date of the assignment and not paying attention to the contoured availability.