I just did this and spent some time thinking through how. I was uncomfortable with missing the target when drilling the hole from the bottom so I looked for another way. What I did was drill a very small pilot hole (#45 or smaller) into the C805 as called for in the plans. Then I put it in place with the locking block into F882. Once I had it centered and where I thought it should be, I took a dental pic that I bought at Menards for about $5 and put it into the hole marking the proper drilling location on F882. Then I took it all apart, took off F825, removed the rib F882 and located my mark. I then took a #40 bit and placed it through a dimple hole in the rib which allowed it to hit the mark I had which has to be approached from an angle. I turned it by hand until I had a perfect start to a hole. Attached the drill and finished the hole. Put it all back together and have a perfect fit.