Lol
Why did you pour concentrate into expansion tank? Its an open system that can be emptied without problem.
Its the engine thats the problem.
So lets assume you got the total volume of coolant as 6 litres including expansion tank to max line
Now we measure how much liquid the tank takes, lets assume it holds 500 ml total, so it would hold 250 ml of concentrate only.
This 250 ml is subtracted from the coolant concentrate that has to be filled in the engine. (Which was half of thetotal capacity)
Assuming system was 6000 ml, we subtract 500 ml from this which yields 5500 ml,
If we halve it we get 2750 ml.
Now to adjust your concentration in system you have to drain about a litre out (save it) then add about 400 ml of concentrate (which is in the expansion tank) and top up the rest with the drained coolant you saved. (Measure the expansion tank capacity to get the exact volume of coolant concentrate that is missing from engine, as you mentioned 800 ml, I wrote 400 ml concentrate shortage in engine)
Now adjust the coolant concentration of the drained coolant with the remaining little concentrate you have and pour that in expansion tank (check total volume of remaining liquid mix and adjust accordingly with your concentrate) - you have a 38% mix in system right now if you need it for calculation.
Its quite easy, i have to do such every few weeks on some random car as someone or some workshop screwed up the concentration levels.
The adjustment is quite easy as you are dealing with a 50% mix, you can use the “sola aana hisaab” or the “aashari hisaab” as you wish. 
