The proper way to measure chain slack/tension is to roll the bike slowly forward with the clutch engaged. When the piston comes into the compression stroke the top chain run will go slack. Go to the middle of the chain, between the front and rear sprocket. That's where you measure the slack. How far can you lift or lower the chain? That's where you want that 1/2 to 3/4" of slack, or free play in the chain.
As for removing links; you can't remove less that two links unless you resort to using a half link. I read where you said you couldn't find the master link. Being that you didn't build the bike it might well be that the original owner didn't use a master link. He could have simply removed links then put the chain back together again in reverse of how you break, or seperate one. By removing pins then driving them back in.
Tom