If you're asking about calculating Gyro drift on a DI, here is how you calculate it.
Table for calculating drift in the northern hemisphere (reverse the signs for southern hemisphere):
Earth Rate - Minus
Latitude Nut - Plus
Transport Wander East - Minus
Transport Wader Wast - Plus
So if you have a DI with a Latitude Nut set for 40 Degrees North and you're flying at 240 knots groundspeed on a westerly heading at latitude 50N you would carry out the following.
Latitude Nut correction for 40N (a plus sign for the latitude nut set for the northern hemisphere):
+15 sin 40 = +9.64
Then you have the Earth Rate which is minus in the Northern Hemisphere:
-15 sin 50 = -11.49
Then for the transport wander, a plus sign for flying in a westerly direction in the northern hemisphere:
(Groundspeed / 60 * tan mean latitude that you're flying)
240 / 60 * tan 50 = +4.77 (plus because you are flying west)
Put them all together:
+9.64
-11.49
+4.77
= +2.92 degrees drift per hour (increasing in reading)
In JAA questions they sometimes give you a random drift error as well (giving a + or - as well) so you'd append that to the final answer. Be careful as sometimes they ask you what is the drift for part of an hour instead of a full hour... (I've been caught out in the feedback quesitons a couple of times).
Thus as you were flying 270 degrees (west) after an hour it would have drifted to 272.92 degrees.
Um I'm still doing my JAA ATPLs so I may be wrong... (hope not as I'm taking the exams soon and I'll be in major trouble if I get this wrong!).
By the way I don't think I would have known many of the answers to the question when I took my JAA PPL (CAA back in those days) seven to eight years ago.
Best wishes,
Charlie Zulu.