As Mick, 3/8" total at the tightest spot. With one finger push lightly down till chain is taught (not bar tight) then using just enough finger pressure push upwards to get the chain taught.
Not wanting to teach Grandmother to suck eggs but...
Also a good idea to check the condition of the lower gearbox pivot bolt hole. If the bike has been used with this bolt slightly loose the hole can 'oval'. Result is that you set the primary tension, tighten the top and bottom gearbox bolts up and then after a ride find the primary chain is bar tight. This is because the pulling force of the rear chain has pulled the bottom of the gearbox backwards along the ovaled hole. Drilling out the case slightly and fitting a piece of thin wall steel tube sorts that out.
Another point is the top adjuster. If you overtighten the primary then it's normal to wind the adjusting nut backwards a bit to give a little slack, then tighten up top and bottom through bolts. You should then nip up (screw in) the adjusting nut against the bracket. This stops the gearbox being pulled forwards by the force of the primary chain and slackening the chain. A small movement in the gearbox can make a big difference to chain tension.
Oh and always check rear chain tension afterwards and don't try and adjust primary with a tight rear chain.