The arc is very important - and how you set it and on what satellite you do your original alignment.
Mounting pole/bracket must be dead vertical - use a spirit level then check with another. Then everything about your dish must be perfect - even the LNB arm, etc. The slightest misalignment and your screwed!
Then set your polar mount, etc, and any motor or jack arm to the 'middle' point (easier to do than explain) then find a bird around 1W and 'peak' for max signal either on a satellite install meter (a proper digital one not a useless meter) or good receiver, and then check the arc. You should be able to scan and find most satellites. Then go for 42E and see if its there. Same goes for 30W. You should find them - and if not, just do the slightest adjustment each time. Finally you should find all sats from 30W to 42E. Obviously this all changes depending where you are, what you can see in the sky due to footprints, etc, but you should have no real problems.
Few hours work and you're done - took me about that long and an extra hour just tweaking and checking.