To create a mitered corner like that, start with a cube.
Set the dimensions here:
Apply scale type Ctrl A. This step is important read this.
Enter edit mode.
On top Ortho abview select the top left vertices and move them 0.026m on the X axis. To make precise movements use the numeric input features of blender: you can type directly the amount you want to move and in which axis, like this:
(Meaning: grab/move 0.026m on the X axis)
Select now the vertices on the bottom right and Extrude 0.068m (or however much you need, since you omitted that measurement) on the Y axis
Switch the pivot point to Active Element, select one of the vertices on the left side to make it active and select the rest of the top vertices, and then scale to 0
In order to see measurements on the 3D viewport in edit mode enable Mesh display->Edge Info->Length
Note that the units used will be those that have been set up on the scene-units section.
(form the Blender Manual)
You can mix units with numbers; for this to work, units need to be set in the scene settings (Metric or Imperial).
Examples of valid units include:
2.2mm + 5' / 3" - 2yards
Note that the commas are optional. Notice how you can mix between metric and imperial even though the display can only show one at a time.
Read on unit names here