Can you be more precise than ‘blew 72 at the roadside, blood 3 hours later’..... was it 3 hours? Or 3 hours 10 minutes....or 2 hours 53 minutes...?
If it was 3 hours you may well be just over IF the roadside test was accurate, which they are not guaranteed to be.
A back calculation is just as curiousGeorge123 described, the ask the laboratory “OK, so her reading was 70 in blood, what would it have been 3 hours ago at the time of the accident?” To do this they have to ask you about what you had to eat, drink and when and your height, weight etc. Some forces seemed to do this routinely a while ago but now it is generally only done after a serious Road Traffic Collision and they don't want the time elapsed to be used to get away with causing nasty injuries to someone else.
There will be no point in phoning for the result. It takes about 6 weeks normally for it to come back and under the current climate that could be longer. You are only likely to get the result from the officer in the case, not from phoning 101. The problem with phoning for the result is that there could be a ‘cock up’ and your result (whilst over) gets filed and the police do not raise a summons within 6 months which they are required to do. By phoning you jog the officers memory, the over the limit result is found and you are prosecuted BECAUSE you phoned up! This does not happen often, but don’t risk it.