Timing

So if my initial timing is say 20*, i have an 18* stop in my distributor. How do I keep it from total 38*? I know everybody is different, every engine isn't the same. Everything I have read is 34-36* total. I must be missing something (not understanding). I completely know how to set total on a locked out distributor. I haven't looked into it but can I buy a larger stop to lower total advance?
yes my MSD came with a bunch of different stops and a bunch of different Springs basically it was a little circle piece of metal and they're colored as well as the Springs. To give you a different amount of total timing..