Nick,
The Imperial figure a Naval Architect would use for mild steel is 490lb/cub. ft. Knowing that, you can calculate the thickness for any given steel weight. 8lbs/sq.ft. is indeed for all practical purposes 0.2" and 10lb/sq.ft is 0.25".
Having said that, the only people for whom this is relevant are shipbuilders/repairers who want to quote on a job or for naval architects performing stability calculations. At our scales, I would argue that differences in plate thicknesses are marginal and, I would suggest, that in most cases 'if looks right, then it is right'. Trust your instincts and your eyeballs.
Hope this helps,
Barry M (Definitely not Victor Meldrew)