Keep in mind that at some point, turning the RF gain any higher will not have a positive impact on the signal-to-noise ratio (S/N). Beyond that point, increasing the RF gain may seem like the signal is increasing, but the noise floor is also increasing, possibly at a greater rate, ultimately degrading your S/N.
In order to find the “sweet spot” that you have in mind, you should turn the gain all the way down to 0 dB and tune to a source like an FM radio station (depending on your antenna’s designed frequency and environment, you can pick another source as long as the transmission power is relatively stable). As you increase the RF gain slowly, your S/N should increase, but around ~8 dB* (for RTLs), you will notice that the S/N stays the same (noise floor increases with signal).
This is the value your RF gain should be set to for optimal results (maybe slightly higher to ensure you haven’t underestimated the value of the “sweet spot”).
*This value varies from system to system, so experiment as instructed above.