If you leave your rakers at 0.025", as the cutters wear and the gullet gets wide the cutters simply cannot bite as much wood because the angle the cutter presents to the wood decreases. The way around this is to progressively drop the rakers as the gullets get wider. Nominally new stock 040 and 3/8 chain has a gullet of ~0.25" and a 0.025" raker depth (i.e. 10:1 ratio) but when the gullet is 0.4" the rakers should be set to 0.040" and when the gullet reaches 0.5" the raker depth should be 0.050". etc
This will provide the same angle of presentation by the cutter to the wood all the way through the life of the chain.
This method of setting rakers is called progressive raker setting A 10:1 ratio corresponds to a raker angle of 5.7º (see
this thread for all the grimy details) but this is in fact quite a lightweight angle for an 066 running a 36" bar. A few year back Mntgun was running ~9º rakers with his stick 066 in your softwoods - this corresponds to a raker depth setting of 0.04"! for new chain. Mtngun found this was his optimum angle with significantly improved cutting speed. Now I recommend you don't do that straight up front but slowly increase the raker depth until the chain starts to grab to the point where it feels like its going to stall, then swipe the cutters a few times as this will back off the raker angle and you should find the chain will cut better than new until the cutters start dropping off.
Have a read of the thread I linked to above and read what CS users say when they try this method out - they simply cannot believe how much difference it makes.
FWIW I use 6.5 degrees on my 3/8 semichisel on the 880 with a 43/60" bar. This corresponds to a new raker depth of 0.028". On my 441 with a 25" bar with lopro I use 7.0º which corresponds to a new raker depth of 0.031". Bear in mind I'm cutting wood that is much harder than what you guys see. I'd be running significantly higher angles if I ever got to mill softwoods.