A calibration file is better than nothing and will at least get you in the ballpark.
There are a few definitions worth knowing about:
"Average" is what most analog and digital meters use, and this is an intregral approximation of true RMS. The problem is that "average" can be almost anything depending on the manufacturer. Cheap meters like the RS just hang a capacitor across the signal line from the mic preamp to slow down the meter movement. "Fast" and "slow" settings just use a different value capacitor (and that capacitor usually is electrolytic with +/- 20% or more tolerance).
"True RMS" is an absolute value which represents the energy in a waveform as defined as the heating ability of the waveform. In early meters, this was done by actually heating a filament within the meter and reading the temperature with a thermocouple. In modern meters the value is calculated mathematically. The RMS value of a sine wave is very close to .707 of its peak value (or another way of looking at it is the peak value is 1.414 of the RMS value). The true RMS value of an an AC wave has the same energy as DC of the same voltage. The RMS value of complex waveforms is much more difficult to measure, even with modern instruments. True RMS meters measuring transient waveforms are still subject to the variances of the ballistics of the meter.
"Peak" is also an absolute value, and as its name suggests is the highest instantaneous value of the waveform at any given moment. It has no relation whatsoever to "average" or "true RMS" values. It is difficult to measure the true peak value of a waveform with an analog meter as there will always be a ballistics lag unless sample and hold circuits are incorporated to hold the peak value for enough time for the ballistics of the meter to catch up. With purely electronic meters (such as your software), true peaks can be more easily read. Even in these instances however the software incorporates a sample and hold in order to freeze the actual peak value so that your eye can register it (or the peak number is held until a higher peak value is sampled). It is also important to know if the software algorithm is performing full wave rectification of the waveform so that both positive and negative peaks of as little as a single cycle (Hz) can be captured and held. The human ear does not really respond to "peak" SPL; the way the ear responds is more like the "average" value.