[twowheels]

Typically I measure the chain slack with the bike on a stand and the rear wheel off the ground.

Because of the relationship of the countershaft to the swingarm pivot the chain is loose at this point, gets tightest when the c/s, swingarm pivot and rear axle are in a straight line, then loosens as you get deeper into the travel.

In theory you could run the chain taut at the straight line condition described above, but you've got to account for mud and debris that may be on your sprockets as well.