It depends which insulins you are using too.
Novorapid for correcting a high blood level is normally set as 1 unit brings you down by 3mmol.
However, this does depend upon age, activity, time of day, how high your levels are etc.
If you were say at a level of 9mmol... and gave 1 unit of fast acting insulin then it is expected that the average adult would come down 3mmol to a reading of 6 within 5 hours.
Please note this is not necessarily right for you.
To test you need to be high. Not to have ate or given any insulin for 5 hours. Then inject on a 1 to 3mmol basis. Check hourly and see what your levels are 5 hours later. Again..this 2nd 5 hours is no eating/injecting. Test hourly at least. If you go low you must abandon and eat.