No the standard practise for OW level divers is to leave the same amount in reserve for all tanks - just means less time underwater with a smaller tank.
Reason being - your safety stop is the same length so same gas requirement and the reserve is for the exact same emergency contingency so why would you leave that short?
Wrong, wrong, and wrong!!!
(Tank) Rated Vol / Working Pressure = cf/psi
For an 80cf 3000psi aluminum tank 500 psi ≈ 13 cubic feet (12.9 if you want to be picky).
77.4/3000 = .0258 then 500psi X .0258 = 12.9 CF
For an 80cf LP tank with a working pressure of 2640 it's .0303cf/psi.
13cf/.0303cf/psi = 429psi.
In other words 429 psi in an 80cf LP tank is the exact same amount of air as 500 psi in an 80cf aluminum. Has absolutely no bearing on safety stop. 13cf is 13cf unless Boyles Law has been repealed.
Other formulae you might be interested in:
cf/psi x psi used = cf used
cf used / dive length = Raw SRMV
raw SRMV / deep or ave atm = deep or ave SRMV (atm = (depth/33) + 1)\
(Target Depth / 33) + 1 = Depth Factor
SAC x Depth Factor x Dive Factor = RMV
RMV x Dive Length = Air Required
Useable Air / RMV = Allowable Dive Length
Definitions:
SAC = surface air consumption (measured at surface at rest).
RMV = respiratory minute volume
SRMV = surface (calculated) respiratory minute volume
Dive Factor is an estimate dependant on temp,current, work rate, etc. and varies with each dive.