The power in dBm is the 10 times the logarithm of the ratio of actual Power/1 milliWatt.
dBm stands for “decibel milliwatts”. It is a convenient way to measure power. The exact formula is
P(dBm) = 10 ยท log10( P(W) / 1mW )
where
P(dBm) = Power expressed in dBm P(W) = the absolute power measured in Watts mW = milliWatts log10 = log to base 10
From this formula, the power in dBm of 1 Watt is 30 dBm. Because the calculation is logarithmic, every increase of 3dBm is approximately equivalent to doubling the actual power of a signal.
There is a conversion calculator and a comparison table here.
There is also a comparison table on the Wikipedia english page, but the value it gives for mobile networks is a bit off.
Your actual question was “does the – sign count?”
The answer is yes, it does.
-85 dBm is less powerful (smaller) than -60 dBm. To understand this, you need to look at negative numbers. Alternatively, think about your bank account. If you owe the bank 85 dollars/rands/euros/rupees (-85), you’re poorer than if you only owe them 65 (-65), i.e. -85 is smaller than -65. Also, in temperature measurements, -85 is colder than -65 degrees.
Signal strengths for mobile networks are always negative dBm values, because the transmitted network is not strong enough to give positive dBm values.
How will this affect your location finding? I have no idea, because I don’t know what technology you are using to estimate the location. The values you quoted correspond roughly to a 5 bar network in GSM, UMTS or LTE, so you shouldn’t have be having any problems due to network strength.