With the DTC-100 you can't just go by the number.
When I had one it would work great at times with signals in the 50-60 range. It would fail sometimes with signals in the 80-90 range.
I found that my minimum reading was about 28 and I never got a usable picture with readings below 50 but I think other people have found readings in the 40s to work.
There are 3 things involved:
1 The meter reads error rates, and signal strength combined. I found that a really solid good signal would read very high like 90 and fail when it dropped to 80 due to error rates. A very weak station might work perfect at 60 but fail when it dropped to 50.
Only the error rate really matters to the tuner. Stronger signals can make it read higher on the meter but that may or may not mean the signal is better.
2 The DTC 100 is very sensitive to dynamic multipath. This could be from aircraft, or from atmospheric bounce conditions. When this happens the reading bounces up and down a lot, and this usually leads to visible glitching. So unstable readings are bad, no matter what the actual number, and the faster the bounce the worse usually.
Atmospheric bounce problems are worst for distant station like over 50 miles away.
3 Multipath can cause glitches in the picture that may not show on the meter. The meter shows strength and raw error rate. But some errors can be corrected by the tuner and others can't so this effect is only visible in the picture not in the meter. A high number of errors that are correctable is preferable to a lower number that are not correctable.
To fix your problem you need to figure out what is causing your bad picture. Weak signals you may need a bigger antenna or a preamp or a better preamp.
Strong signals you may need an attenuator.
Multipath you may need a more directional antenna, or a slight tweaking of the aim.
If you are 70-80 miles away you are lucky to see anything and if you want it better you may need to mount your antenna much higher up.
If your station is new, you may want to check with them too, and make sure they aren't causing the problem.
Finally if you determine that dynamic (ie changing) multipath is your problem say due to being close to an airport: About all you can do is use a very directional antenna and change to a tuner that handles dynamic multipath better. That is why I had to sell my DTC 100 and switch to the Samsung tuner.