habilis

Sounds like you have the crank on hand already. If so, you can determine where the square tapers of the BB spindle should end. It will be at the bottom of the recess for the crank bolt, or very close to that point.

You also know that the center cog of your cassette will determine distance of chain line from the center of your hub (midpoint of over-lock-nut distance of your rear axle). On a triple, the middle ring of the crank should be the same distance from the center of the BB shell. All of these measurements can be taken from the bike and the crank, so there is no guessing needed.

Once you know the length of the BB spindle from the center of the BB shell, double that distance to get the overall spindle length of a symmetrical BB.

To avoid confusion, I make a simple top-view diagram of the drive train and label the distances.

[Edit: Since you are converting your triple crank to a double, the manufacturer's recommended BB length is irrelevant. You must measure CL from the space between the rings to the midpoint of the BB shell after you reconfigure the crank. Add to the CL the distance to the bottom of the bolt recess. Double that total to get BB spindle length.]