As a musician myself, I'm a bit confused. The example you gave above (using A, B, C, D, E, F#, G) would indeed be C Lydian (4th mode), but would also be A Dorian (2nd mode), B Phrygian (3rd mode), etc. Wouldn't you want to identify the mode that is native to that key signature? One sharp (F#) is G major/E minor, right? I guess you could list all of the possible modal combinations for each group of 7 notes. Seems like what you really want is a scale/chord calculator. Here's a couple I made for lessons4guitar.com:
Click Here
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