First single speed project - help!

40t sounds quite light, especially if your riding it freewhell, it kinda depends how much cyling you've done/how fit you are...

I am in pretty good shape, I would say. But I'm not planning to ride long distance, just short commute. I was thinking about a slightly smaller front cog, as I want to try fixed gear, want it to be manageable, not too difficult to stop.

use the placement for the middle ring.

middle ring?
single speed = one ring. No?

Not sure why chainsets had what bb they would match, pretty much all chainsets fit all bb's, unless you get some really old or obscure kit...

I guess it was more a recommendation "BEST AXLE LENGTH FOR THIS CRANK IS..."
I guess I am mainly concerned with the chain alignment, as obviously the axel length will affect the chain position.

I'm sure your local bike shop will measure the bb.

Yea, but the will be expensive. + I want to learn...