You want the center of the coil to be in the center of the top plate IIRC.
I tend to get it as close as possible by eye (Alot of test fiting and moving the coil by hand)
CA glue is what you should use ( Dave, fixmyspeaker.com has some for sale).
I personally havent built a dual spider sub, just used a single stiff.
1.) Position Coil and mark on former where spider sit.
2.) Mock up cone onto former. Remove once you ensure a good fitment.
3.) Shim Coil in gap at the correct point. Make sure the spider sits naturally and square. Glue Spider to former.
4.) Drill cone, and feed tinsel leads into cone, leaving enough slack for movment obviously.
5.) Glue cone to spider/former. Make sure you have a STRONG tripple joint as these often fail under alot of stress.
6.) Solder Leads to VC leads
7.) Glue down Leads to prevent movement
8.) Leave glue to dry overnight
9.) Once glue is dry, remove recone assembly and make sure the gap is clean, and all landings are preped
10.) Reshim the recone, and glue spider landing. Add weight and spray activator.
11.) Repeat for the Cone. Once it is dry, you can remove the shims.
12.) Glue Dustcap onto cone.
13.) Solder Tinsels leads to terminals.
Remember, Im not a pro so it can be wrong. I am a hobbiest who plays in my free time.