I think you've had input from JasonWho on your bike?
He's familiar with your 'vintage' bike. He can tell you the basics (jet-wise for sure) on the Mikuni.
As Jag said, parallel generally works. You will likely see mold lines on the floats that will make it easy to determine when they are parallel.
Although the Mikunis on my R5s/RDs had round floats with no mold lines. Kind of hard to get a round float parallel to the carb body. Actually, they could be considered parallel to ANYthing you wanted to compare them to. That would be helpful......