Zo, Over the years it's been recommended that about a 2:1 ratio of test to deca be used. But in all reality there's not a specific science to it. It appears that most maintain a better libido using that ratio, but I for one (and others I know) have broken that "rule" often and had no issues.
Everyone has different sensitivities. Some get the puffy nips from very low amounts of androgenic compounds (just a "for instance". I'm not calling deca an androgen). Some get bad aggression, acne, etc.
With deca, some get "deca dick" and lose libido while others see an increase in libido and have hardons all the time. It's good to have that 2:1 ration in mind, but until you experiment to see how your body handles it, I suggest not getting stuck in the opinionated "rules of use". . . I hate that box!
All that blah blah blah aside -- 200mg a week of test and 200mg deca may be perfect for you. Try it and see.
As for minimum dose that deca will help joints -- as little as 100mg can be uselful longterm.
I'm not too up on the current studies, but sometime back I was reading some research (legit medical studies - not forum opinions) about hrt with test and deca for lifetime use. I believe that study was researching an effective male contraceptive however. But it definitely wasn't a contraceptive for myself. I was deep into a test/deca/dbol run when I concieved my daughter.