you can’t correlate the rxlev with the position of the MS.
rather, you must use the Timing Advance, which is an information available in the Measurement Result messages.
The TA gives you the distance between serving BTS and MS.
Since you can’t triangulate the MS thanks to the neighbours, you must know what is the orientation of the serving cell’s antenna.
Based on this, you are able to know, roughly, where is the MS.
The RxLev of the neighbours could then be used to further optimize the position of the MS, but I don’t think it that’s a good idea (RxLev depends on many more things than just the longitude and latitude of the MS !)