Having just had a long drawn out struggle with specifying an equation in parameters/dimension edit boxes and reading through others similar problems with the intricacies of getting a formula to work, I have this idea: The internal workings of Inventor are obviously using a subset of unit variables. The main problem seems to be that you want to use data drawn from various model and user parameters with differing units. The box where you want to use the calculated value is programmed to accept only units of a certain type (unitless included). When you enter a formula the calculated value is often rejected because the output units are not compatible with that particular box - unless of course you know the precise syntax, which is difficult to get a handle on especially if you are in a hurry. This leads me to a lot of frustration and judging by others posts I am not alone. If the formula is wrong it turns red and if you hover over it Inventor tells you exactly the mish-mash of units that are incompatible. This is helpful but not the easy solution because from there you still have to battle with the syntax to try and get it to work. Where a number is calculated despite wrong units, why not program Inventor to give the user a choice at that point, say a pop up RMB menu 'Use this value but convert formula to units: mm in rad deg ul etc. then reformat the formula automatically to give those units.
