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I understand that a preamp only helps with loss through the cable run and does nothing to benefit reception.
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Actually, that's a mis-statement of how it works, it's pretty much an over-simplification.
A low-noise pre-amp's input noise figure would substitute for the combined insertion loss of the downlead and of the tuner (plus any other losses). Adding one, where appropriate, can extend the noise floor limit by a few dB, perhaps even as much as 6-10 dB for a typical tuner and preamplifier with 75' of RG6. IOW, very weak signals that would get lost in the cabling and tuner's input would have sufficient power to be above the tuner's noise floor and then may possibly be received (there are other factors..).
To do the actual calculations, you'd have to use what's usually called a "Cascaded Noise Figure" calculator or the equivalent to find the "System Noise Figure" when using (or not) a preamplifier.