Here is my answer to the PM I received on this, as I figured others would be interested....

The internal volume of an 88 CF SCBA tank is 550 CI, it holds 88 CF at 4500 psi.... The standard 6000 psi steel cylinder is a DOT 3AA-6000, and they have an internal (water) volume of 1.50 CF.... and are typically listed as 480 CF (sometimes called a 510 CF).... 6000 psi is 414 bar, so using Boyles, that should be (414 x 2383) / 1728 = 570 CF.... Using VanDerWaals for Nitrogen instead of using 414 bar, we use a factor of 322 (339 for air), so at 6000 psi it should contain (322 x 1.50) = 483 CF (509 CF for air).... so that 480 (510) CF is pretty close.... If your Nitrogen tank is a different size, or not filled to 6000 psi, then the calculations below are wrong....

The problem is, that both tanks contain fewer CF of air (or Nitrogen) between your fill and refill pressure than a linear relationship would suggest, due to the VanDerWaals effect.... The only real way to do this is to use the density of Nitrogen at the three pressures (6000, 4500, and 3000)....

Density of Nitrogen at 6000 psi = 376.1 kg/m^3

at 4500 psi = 310.1 kg/m^3

at 3000 psi = 225.1 kg/m^3

From that, we can calculate the weight of Nitrogen released from the large tank (assuming 1.5 CF internal volume)....

1.50 is 0.0425 m^3.... so at 6000 psi it contains (376.1 x 0.0425) = 15.98 kg. of Nitrogen.... and at 4500 psi, (310.1 x 0.0425) = 13.18 kg.... The difference is (15.98-13.18) = 2.80 kg. of Nitrogen released for a drop from 6000 to 4500 psi....

550 CI is 0.00901 m^3.... so at 4500 psi the SCBA tank holds (310.1 x 0.00901) = 2.79 kg. of Nitrogen.... and at 3000 psi, (225.1 x 0.00901) = 2.03 kg.... The difference is (2.79-2.03) = 0.76 kg. of Nitrogen required to fill it from 3000 psi back to 4500....

The number of fills is the weight of Nitrogen released from the big tank, divided by the weight of Nitrogen required to fill the SCBA tank.... (2.80 / 0.76) = 3.68 fills....

Kudos to you for coming up with 3.5 fills....

Bob