Attach method dictates edge distance requirement
I've spent WAAAAY too much time on this gear tower modification. I appreciate everyone's input because it got me asking questions.
For those who are making this modification it seems that how the cover plate is attached matters. The reason it matters is how the plate is attached dictates the necessary edge distance of the screw. There isn't a ton of room in the tower itself to manipulate edge distance because there is a angle underneath that is 9/16" from the edge of the opening. This has to be avoided which dictates where the hole can be drilled.
I see three ways to attach the cover plate: 1) Flat screws. 2) Dimple with flush screws. 3) Countersink with flush screws.
It seems that each of these has a different minimum edge distance requirement. Military specification document MIL-R-47196 lays out the edge distance requirements for overlapping materials. The plate is called the visible piece and the tower is called invisible piece as it sits underneath. Each has a different edge distance requirement depending on which one of the three methods is used to attach the plate. This speaks to rivet sizes and not screws/bolts, but the logic should hold (I think).
Based on my measurements the largest edge distance available in the tower is about 6/16" (.375). This provides maximum edge distance to the opening and allows the nut plate to clear the angle underneath. The plate edge distance can be more than that as long as it stays inside the external rivet heads on top. For a 5/32" rivet (.156) the document calls for .281 - .344 minimum edge distance - 9/32" to 11/32".
The net is, doing just a flat head screw requires the smallest edge distance. Dimpling or countersinking will require more than 11/32" (.344) of edge distance (for a 5/32 rivet). For reference 6/16" of edge distance is .375.
Since the document doesn't give an edge distance for a #8 screw, I found a forum that the aerospace engineers use. They don't definitively answer the question and they don't like "rules of thumb", but they offer up an edge distance formula of 2 times diameter + .05. That would be .378 for a #8 screw. If we have 6/16" to work with that is .375(close enough?). The forum link is below.
Hope this helps for anyone making the modification. None of this speaks to which method would best carry the loading forces. I'll leave that up to the engineers in the group.
https://www.eng-tips.com/viewthread.cfm?qid=153469