The tightest point for the chain as the swingarm travels through it's arc is when the countershaft sprocket, swingarm pivot and rear axle are all in a straight line. Get a buddy to put his chest (her?) on the bike's seat , grab the swingarm, and pull down untill all three are in a straight line. Check the chain tension at this point (you'll want a little play) and adjust to suit. Having the chain too tight stresses the components badly and, for example, will lead to tell tale cracks around the rear motor mount where the swingarm bolt passes through.