A log scale here feels much more intuitive to the average user than a linear scale, as long as you don't tire them (consumers) with dBm details.
If your range is -100 to -40 dBm, I'd map -100 to a small percentage, say 5% and -40 to 100%. That is assuming you can still receive a signal at -100dBm and you have a separate indicator that your receiver is not locked => 0%
So when locked:
$$percentage = \dfrac{19}{12}×register + 163\dfrac{1}{3}$$
When unclocked:
$$percentage = 0$$
What you are looking for is a receiver with an RSSI function (Received Signal Strength indication); this is usually an analog output proportional to the log of the signal strength.
The datasheet does not give particularly good specs, as is common with the Sparkfun datasheets I have looked at. If you could identify the real manufacturer, they might have better data.
In this case, that takes us to a page on www.rf.net.tw which has exactly the same data sheet. However it does have an "Ask Question" form where you could ask if this module has an RSSI function. (Be prepared for the question to be ignored or misunderstood, but it may be worth a try).
The datasheet DOES say "Modulate Mode : ASK" which means "Amplitude Shift Keying" which means the linear output MAY vary in voltage to some extent with signal strength, but probably not enough to be useful, because AGC (automatic gain control) will keep the output signal almost constant.
One of the comments on the Sparkfun page talks about using the linear output for RSSI and links to this page about it. Doesn't look very reliable to me.
This page describes digging into a similar module (different manufacturer) and finding an undocumented RSSI signal, if you are brave enough.
All of which probably leaves you looking for another receiver : this time, search for "315 MHz receiver with RSSI" and you may find something more useful.
Best Answer
Andy is correct - we really need to know the details about frequency, power out etc. to give a precise answer but as a general solution you will require a pick-up/tuned circuit (LC), a sensitive detector (diode) and some sort of buffer/amplifier (possibly a FET/mosfet/op amp) and a meter to indicate the field strength.
Accurate calibration of what the meter reading actually means is of course a different matter but if you just want to detect a particular field strength for proximity you could always use a comparator to set the point at which the field strength will exceed a certain value of voltage output.