seems like an on topic question. Any way to get a rough estimate of weight by measuring using length, girth etc...? For example generally a 20 inch fish will weigh X amount of pounds and above.

Yep,

Print this out and keep it around:

http://www.dec.ny.gov/outdoor/9222.html

20" Smallmouth = 3 lb 14 oz

20" Largemouth = 4 lb 10 oz

was thinking 2-2.5

Instead of just guessing...  You can find out exactly what your fish weighed.  If you still have that spring scale, take some weights and keep adding them, till they reach the same amount as your bass that you caught.  Then take those weights off and then put them on your digital or another scale that is accurate.  That should give you the exact weight, and you wont have to keep assuming.

For example.. I was fishing sat and I caught a nice fish.  My scale was having some serious problems and wouldn't read correctly.  I weighed a couple fish and digital scale read .23kg.. then I put on my big one and it read .44kg.  I know that if you convert kg's into pounds, that would be about a pound or so.. but my scale was being really screwed up.  After I was done fishing, I came home and started adding weight until it read the "exact" weight of my bass I had caught earlier.  I then put that weight on a calibrated doctors scale and it came out a tad over 5 pounds.  Problem solved.