Gas Mileage???

OK, gotcha
050 +15 is not enough
Most fast-rate hydraulic flat-tappet cams are about 40* to 44* less from the advertised to the 050 number. A slow cam might be up to 60degrees. Only a solid will get under 40* .
Then you have to consider the lobe shape. I would not use that method for any calculations if it would be 5 *out, that would be a huge difference.
As an example ;that 256 cam above is advertised at 256/268/110 at .006 lift, and the 050 numbers are 212/218. So that is 256 less 212 =44 degrees on the ramps. The recommended install is 106*. That puts the intake closing angle at 54* ABDC
Now to use their method; adding 15* to 212* =227* So that absolutely will not work. But if you assume that their programming adds another 15* on the closing side that we do not know about. then adding 2x15* to 218 = 248*. And that is still 256 less 248 = 8* short. Dividing that by 2, puts the ICA out 4 degrees.
Doing the math on the previous 11.3 teener, the new Dcr is 9.78/206psi. That is an increase in Dcr of 9.78 less 9.57 = .21 and 6 more psi.
So I never use that method.