Signal quality is more important assuming there is some signal strength if you are to aviod constant drop outs and blocking.
If your getting a stable picture, tweeking the dish won't give you a better picture. Digital transmissions are basically you get it or you don't, not to be confused with old analogue aerials.
Normally I'd expect to see at least 50% or quite a bit more (75%) but the standard sky dish and LNB is a bit limited in the adjustments available, a proper dish and LNB will always give good results assuming it is pointing at clear sky. Trees, walls, buildings, and even boiler flue condensate can screw up the signal. A poorly fixed dish that can move will also bugger things right up, these dishes need to be spot on really, just lean on a dish and see the picture disapear completely.....
Of course your 40" LCD might be the limiting factor to your current PQ

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