Using setting circle for Right Ascension I'm a little bit confused about the setting circle of my EQ5 mount. Basically, I'm trying to find M42 nebula using Sirius star as reference. So, considere that I did the following steps:

A) Polar aligned for south pole
B) Pointed the telescope to Sirius star
C) Adjusted the RA wheel to 6h45m and DEC wheel to -16º42'


Now, I want to view the M42 nebula basing only in the RA and DEC wheels (without using the finderscope or whatever). 

Then, I did:

D) Changed the coordinates in the EQ5 mount to match RA 5h35m and DEC -5º23
E) Spent 5 minutes performing the step above (D)
F) Looked in the eyepiece


So my questions are: 

1) After doing all these steps above, will the M42 nebula be 5 minutes away from the center of my eyepiece? So instead of using RA 5h35m I will need to use RA 5h40m?(??)

2) If yes, will I need to perform all the above steps every time that I want to see a new object? I mean, as fast as I can, to minimize the RA error...

3) And if the answer is yes again to Question (2), how useful is the setting circle under these conditions for you?

I will appreciate any kind of help.

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I think I mis-read your question before. Let me rephrase it to make sure I understand. In step D (moving the scope from Sirius to M42), that process took 5 minutes to complete. Or maybe it took 30 seconds to complete, but you then took a 4.5 minute coffee break :-). So during that 5 minutes of time since you calibrated the setting circle, M42 has drifted another 5 minutes to the west.


Yes, the object will be 5 minutes west of the eyepiece. Since your setting circle does not update during that time, you need to position the scope 5m to the west, or to 5h 30m right ascension (not 5h 40m R.A.).
Yes. Before moving to the next object, you should reset the right ascension setting circle to the coordinate of the current object. Then move to the next object. Then compensate for the time it took to move the scope. Of course, switch to a low power eyepiece just before starting the move to increase the chance that the next object will be in the field of view.
Mechanical setting circles are useful for small jumps. I have used them on my scope to locate Venus near the Sun. I have also used the circles on the scope at my club's observatory to locate planets in the day. (That is slightly different that your setup since the club's scope is permanently mounted, professionally made, and has really large circles -- all of which help to reduce any errors.) If I remember correctly, the "rule of thumb" is that mechanical circles should be about the same diameter as the optics in order to be "accurate". A 200 mm reflector (the diameter of the mirror = 200 mm) should have circles that are about 200 mm in diameter. But anything should definitely get you close to the target.