Many ways to do this but the approach that I would use so that it is easier for me to manage and remember 6 months down the road is to control majority of the info from the top level assembly. This way there's only one place to go when you need to update the rules. Also since you are reading an excel file, I would try to read the file once verse multiple times due to each subassembly reading the same excel file. Reason is each time you are reading the excel file, an excel process is started taking memory. So if multiple subassemblies are reading it then Excel is being launched and closed each time. Basically read the excel data in the top level and then push the calculated values into the parameters in parts. I would definitely create user parameters in the parts (thickness, OD, Dia, etc.). This will make it easier to find the parameters from the top level assembly verses d12, d15, etc. Since your parts are in a subassembly, use Parameter(****'y MakePath) from the code snippets to get to the parameters in the part while in the top level assembly. Others can chime in on their approach but from the info that I know, that's what I would do. Hope that helps.
Dan
