Normally I do 10:1...8:1...something like that. I've done 5:1 before with no problems. I know others who go more concentrated....even 1:1. I've found time in a warm bath to be more important than the concentration amount. Sometimes 5-10 minutes...other times an hour or more on simmer. Also you may go more concentrated if your boiling many items at the same time
One thing to look out for is newer rubber. Sometimes it swells beyond being usable or contorts. You may think it's rubber but it's not ......or it's a combination of different substances to emulate rubber. This you usually find out the hard way. Older items are usually rubber and it's safe to work on however for this reason I always do a quicker boiling to see if I'm going to run into problems......before I go on. I've screwed up enough stuff and learned the hard way.
It usually dosent take a long time in the boiling pot to know if your going to run into problems.
Edit....also the cream I use pure...no diluting.