Best is to just count, but I always cut mine to length with a grinder on spot. Are you changing the sprockets as well? I got 15/45 with a 116 link chain. Works good for road use
the length of the chain varies with the gearing. also, what is your preference? a top gear that revs out comfortably? slowly or is does it never get there and it is an overdrive saving the engine?
fact is 16x40 is an overdrive right now. that makes the gaps between the gears huge. I'd be looking at 16x44 and just getting topped out over a mile. that will need a 116L chain that will require a link off and down to 114. they don't sell 114L usually see? 116.....112...etc.
the same gearing will need only a 112L chain and the bike may well be short wheelbase too!