Do include extra links. I just did this conversion to my mtn bike, and my trick was to put the chain around the cog and around the tensioner's roller, making sure the tensioner is roughly where you want it to end up (have someone else hold it in place). Then wrap the ends of the chain to the chainring and note how many overlapping links you get - that's how many you'll get rid of. You'll need about three hands for this if the tensioner is already tensioned (ie you've tension-loaded the spring).

Then, count up the number of times I've used the word "tension" and it's variations, and remove that many links.