Hi Margaux,
From the injection site coordinate [8870,860,792] you provided, I believe you are referring to experiment 263780729.
From the experiment page you can see that the injection site is large and spans multiple cortical layers.
The coordinates [8870,860,792] should be interpreted as point inside of a 0.22 mm^3 injection volume.
While it is possible use AllenSDK (start with this tutorial) to figure out what cortical layer CCF coordinate [8870,860,762] lies in, it cannot be interpreted that the injection site is localized to that layer.