Mechanics and Fixing Any Questions Answered

From that OneUp guide link:

Boost 148 was created to give frame makers more design freedom for tire and front derailleur clearance. From a drivetrain perspective it moves the cassette 3mm away from the frame centerline. The new ideal chainline for Boost bikes is 51-53mm.

To increase the chainline by 3 mm you should buy a BB with spindle 6 mm longer than before. 110 mm before means 116 mm now.

As you have said, the only way to be sure would be to remount the BB onto the old frame, tighten down the crank again and measure chainline. It's a 10 minute exercise including removing everything again. You don't even need to put the left BB cup in. If you find 49 mm then you know you need 52mm chanline for the new bike.

Note: The 68 mm vs 73 mm shell width doesn't make a difference to chainline, only the length of the BB spindle matters.