One has to keep into account that cones and pellets require different extraction times.
Pellets release in almost half the time than cones.
A 30 minutes addition if made with pellets is almost equivalent to a 60 minutes addition with cones. You get all the heavy stuff: the resins, the bitterness.
If your recipe calls for a 30 minute hop addition in cones and you use pellets, use 15 minute for a comparable effect. 30 minutes with pellets is comparable to 60 minutes with cones.
Generally speaking, cones release in 15' half the bitterness than in 60', and at 60' they have given most of what they can give. Pellets at 35' or so have given most of what they can give, and if you want half that bitterness, you should probably calculate 10' or less.
Strong, Brew Better Beer, has a table at page 66 with the cones - pellets percentage of utilization with different extraction times.