Shaw familiar guys can answer better. You say your lnbf is designed for a 75cm dish.
So the feedhorn of the lnbf must be designed to prevent "spillover" to reduce interference from beyond the rim of the dish. If you were to picture the beam angle from the center of the lnbf throat to the dish rim in a V. Your focus is very close to the apex of the angle at the throat.
So. A larger dish won't hurt if it's an offset dish. There are online gain calculators to plug in different dish diameters.
If the larger dish maintains the same or very close beam angle at the lnb focal point, you should be golden.
Things to consider though is if the dish will allow easy mounting of your lnbf. A bracket may be needed.
With that comes you needing to move your lnbf in closer or further away from the dish to attain that focus needed to peak your signal.
Other than that. I always use my receiver and an app that lets me see signal on the receiver from inside of the house.
Can't comment on your signal finder box. But it seems that if the settings on it match what your receiver displays in setup. It should work. Although I do hear that some do have issues with them playing nice.
From the dungeon: