I got a bike recently with kind of a weird set up. It has only one chainring, and a 5 speed freewheel, making it a five speed. I've grown to like this set up and not having to worry about a front derailer, but it has raised some chain length questions for me. I'm pretty sure it's too long right now (since it occasionaly falls off one of the derailer pulleys), and because I took off the chain today to clean the drivetrain, and when I put it back on, I checked with the big/big method. There were about 3 inches of overlap, but I'd run out of time at my community workshop and had to go. Most methods for determining this that I've found sacrifice either the big/big combination, or the small/small combination, neither of which I really want to do.

If I use the "largest cog to the chainring without running through the derailer" method, will I have enough length to run on the big cog? Should i add two extra links instead of one? I have a chain tool and a quick link, so I can do it at home, but I wanted to get some opinions first.

Derailer is a Shimano Altus-LT
Chainring is a 42 tooth piece of crap
freewheel is old and not sure of the tooth count

Thanks