I think the confusion is probably that this is your method alone. SAAMI defines +P as +10% pressure (not powder) rounded down to the nearest 500 psi. Thus .38 Special is:
17,000 psi × 1.1 = 18,700 psi rounded down to the nearest 500 psi becomes 18,500 psi.
For 9 mm Luger it is 35,000 psi × 1.1 = 38,500 psi, with no rounding needed as the result is already a multiple of 500 psi.
+P+ is not a SAAMI standard. It mostly indicates a round intended only for law enforcement and other public sector applications and the agency buying the ammo has to assume all liability for gun damage caused by it. For example, one fellow complained on another forum that the Federal +P+ 124 grain Hydrashok round gave him lower velocity than Winchester's generally available +P load with the same weight bullet. Well, Federal wants to sell that round to law enforcement, and the +P+ label helps them with that, but either they use a powder too fast for the application, or it isn't really saying what the pressure is.
That funny example aside, Father Frog has some posted info for other ammo, and he shows .38 Special +P+ is, by unwritten agreement, 30% over SAAMI standard MAP (Maximum Average Pressure) rounded down to the nearest 500 psi. So, you have:
17,000 psi × 1.3 = 22,100 psi, rounded down to nearest 500 psi = 22,000 psi.
That number is remarkably close to the CIP standard of 150 MPa (21756 psi), which they use for all .38 Special loads. No milder European standard exists. This leads me to believe what the ammunition industry calls +P+ is actually perfectly safe in any modern .38 Special. One might want to treat aluminum frames more gently as a matter of routine for practice, but when the flag goes up, even they will take a good number of +P+ loads without stretching. Especially newer ones using better aluminum alloys than the early ones did.
For uncompressed loads in the .38 Special, it is close to accurate that pressure increases as the square of the powder charge. So, to convert a standard .38 Special load to +P:
√(18,500 psi / 17,000 psi) = 1.04, so, using the same case, bullet, primer and powder, multiply your standard pressure charge weight by 1.04 to get a +P version.
For industry +P+:
√(22,000 psi / 17,000 psi) = 1.14, so, using the same case, bullet, primer and powder, multiply your standard pressure charge weight by 1.14 to get a +P+ version.
9 mm +P+ is not figured the the same way as .38 Special +P+. Instead of 30% over the standard load limit, 9 mm +P+ uses 20% over standard by industry convention. Again, +P+ is not a SAAMI compliant standard, but an industry creation that is a pseudo standard by general agreement and is unique for each cartridge that has it. I'm just including it to be sure it is clear +P+ does not have a consistent definition.
9 mm +P+ is:
35,000 psi × 1.2 = 42,000 psi, and again, no rounding is required as it lands on a multiple of 500 psi.
Using Ddang's 30% increase in powder, if his starting point were a true maximum load of 17,000 psi, he should tend land at about:
1.3² × 17,000 psi = 1.69 × 17,000 psi = 28,730 psi, if he started with a true maximum 17,000 psi load. The original starting load he discussed wasn't quite there, hence the 26,000 psi vicinity result.
28,500 psi is at the lower end of the normal .357 Magnum load range, so the recommendation to put these new loads in .357 Magnum cases is prudent.