There is nothing that says you can't make a calculation, as a matter fact you NEED to make a calculation when you are going to have a combination of conductors. 314.28(A)(3) spells out that you can have a "combinations of conductors" (key words) as long as they are less than the maximum conduit or tubing fill permitted by Table 1 chapter 9, provided the box or conduit body has been listed for, and is permanently marked with, the maximum number and maximum size of conductors permitted.
So if an LB is marked like the above requires with the MAX 3-350, then you are allowed to
have a "combinations of conductors" (key words) as long as they are less than the maximum conduit or tubing fill permitted by Table 1 chapter 9, BECAUSE the LB complies with "provided the box or conduit body has been listed for, and is permanently marked with, the maximum number and maximum size of conductors permitted". Does anyone see what I'm saying here?????????????????????