You can use a table, correctly or incorrectly, you can ask the kid at the building supply to use a manufacturers software for an engineered product such as an LVL, which will yield the same results as the table. The manufacturer will give you the software to do the same thing if requested, the results would be the same as a table. You can do the math longhand or with a computer program that the manufacturers software is based on, which is how their software and the tables were derived... or you can pick some point in that chain where you are uncomfortable, rather arbitrary. For an engineered product the manufacturer supplies engineering support as part of the sale if requested so it's really a non issue there.
For solid sawn materials the AWC, as support for that industry, has come up well short of the mark. Their span tables and calcs are limited to single member beams such as joists and rafters where they could easily publish tables, give away software and provide engineering support for lumber products similar to what the engineered product manufacturers do. I've asked and their response has been that they don't have the funds available. That is actually bad logic, if they provided these services they would find greater use of their products. Quite often in order to satisfy an inspector it is cheaper to purchase an engineered product accompanied by a table or manufacturer's in house stamp than to engage a private engineer to show that a solid sawn product is adequate for the task at hand. This results in a less appropriate, more heavily processed material used where a perfectly adequate, cheaper and "greener" product might be the better choice.
