In the grating equation nλ=d sin θ, the quantity θ will be determined in the lab.
When you look through the eye-piece, the angle of the eye-piece is the required angle. The apparatus has angular graduations below the eye-piece which can be used to measure the angle.
If you are not using the apparatus to view the interference pattem, you can simply use meter
sticks to measure the distances and calculate using geometry/trigonometry.