That is correct. Your strongest TV signals are around -61 dBm, which is not that strong. Signals at this level are not strong enough to be pushing the limits of typical pre-amps.
If you had a strong FM transmitter nearby (200 yards is very very close), then there would be a problem because the FM signal might be too much for the amp to handle. If the nearest FM transmitter is several miles away, then this might not be so much of an issue. If the strongest FM signals are less than -30 dBm or so, then I don't think you need to worry.
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Does that mean I could preamp both UHF and VHF in the 20db - 30db range?
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The main purpose of amps is to overcome any signal loss that would normally occur AFTER (or "downstream") from the amp. This includes things like cable loss, splitters, and inefficiencies in the receivers. Ideally, you want enough amplification to overcome these downstream losses. There's no real benefit to having more amplification than necessary.
For most people, the main things to consider are:
1) Cable loss. RG6 coax will typically lose about 6-8 dB for every 100 feet of cable in upper UHF frequencies. There is less loss at lower frequencies, so we typically focus on the worst case losses that happen in upper UHF.
2) Splitters (a.k.a. power dividers). When a signal is split, the amount of signal energy going down each branch is a little bit less than the original signal. The amount of signal reduction depends on the number of ways the signal is split. A 2-way split equates to roughly a 4 dB loss. A 4-way split equates to roughly a 8 dB loss. An 8-way split is roughly an 11 dB loss.
3) Receiver noise figure. The tuner circuitry in your TV or set-top box will have some inefficiencies that equate to a slight loss of SNR while processing the incoming signal. This reduction of SNR is referred to as the Noise Figure for the receiver. The tuners in most consumer products today (TVs, DVRs, converter boxes, etc.) have Noise Figures in the range of 6-9 dB.
So, if your setup has about 5 dB of cable loss (~60 feet RG6), 4 dB of splitter loss (2-way split), and a 6 dB receiver noise figure, then you need about 15 dB of amplification. A 20 dB amp would be more than enough to handle the situation. More amplification doesn't help, so don't automatically think that higher gain is better.
Also note that amps themselves introduce a bit of noise in the process of amplifying a signal. This equates to a loss of SNR right at the amp, and thus amps also have a Noise Figure specification. A "good" amp should have a Noise Figure rating of 3.0 dB or less. Many of the amps from companies like Winegard, Channel Master, and Antennacraft have good Noise Figure specs.
Beware of amps that have very high Noise Figures or ones that do not publish a Noise Figure spec at all. Some "bad" amps have been measured to have Noise Figures in excess of 10 dB, which almost negates the benefit of having an amp in the first place.
