I'm not sure I get why the math is needed. Mount the tensioner, Put on the cogs and chainrings you plan to use. Wrap the chain around the cog, tensioner and ring for each application. Break chain to suit and fasten with a quick link (that probably came with your chain). Yes, does require a chain riveter to break the chain. Buy, have a shop do the work or go to a bike coop if you have one.
If money is the issue, try asking for a used, junk chain. Use it to determine the length.
Ben - who's been riding single-speed and fixed for 50 years and has yet to calculate his first chain. In fact, who's never known how many links he's riding.