Congratulations, if you are getting a borderline lock on ESPN from 40.5W with the C138 LNBF as shipped you can expect to improve things with tweaking.
A polo pipe or the breadboard T plate will get you boosted into a stable lock. Tweaking with the LNBF/Scalar pairing will get some more boost and possibly some from Az/El alignment.
Well done, you have stuck with it and achieved the 40.5E ESPN lock with a small dish, a feat to be proud of considering it is your first venture into C-Band.
I have attached below the feed-scalar pair table I compiled for 40.5 West some time ago.
For three of the four frequencies I sampled, the best signal levels were achieved with the feed opening aperture between 5mm and 10mm inside the focal length.
However, these readings were for a prime focus dish with a flat scalar and not an offset dish with a hooded scalar so I am not sure how applicable they are to your case?
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