How do you Reckon?
To Reckon yourself as dead to sin is to account yourself as Already Crucified, that is, to account your old man as Already Crucified.
It is Reckoning the Cross of the Lord Jesus Christ to be also the Cross of your old man.
Thus you consider the death of the Lord Jesus to be likewise the death of your old man.
Indeed, you deem the time when the Lord Jesus died more than two thousands years ago as having been as well the Exact Time of the death of your old man.
For the old man having been Crucified with Christ is a Fact, an Accomplished Fact.
In the Eyes of God, the sin factor is Already Dead; and so, we must Reckon ourselves to be dead to this sin factor.
In our Believing from the heart that God will Realize in us what we Reckon, and in our deciding with the will that we have died to sin, we shall no longer be bondservants of sin.