Maybe I'm reading this wrong Adam but it seems you are calculating/adjusting total timing using vacuum advance? Initial and mechanical advance combined (with no vacuum advance factored in at all) will give you your total advance (for ex. 34* total with 12*initial & 22*mechanical). Springs will dictate how soon/fast in the rpm range the mechanical advance comes in (you should be OK as Richie said curve was good). The vacuum canister will further advance the timing at cruise with high vacuum signal or idle when connected to MVA as you have. This will help with engine cooling at idle and cruise (benefit to oil pressure too). The need for the adjustable vacuum canister is to compensate for your cam's profile that gives you a lower max. vacuum than stock type cams. Your lower max vacuum signal may not be high enough to actuate a stock vacuum canister.
Maybe you do have a handle on this concept and I read it wrong but just trying to help.
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1968 LeMans conv. 350 HO - 4 speed triple white (hear it idle here) https://www.youtube.com/watch?v=eVmq...ature=youtu.be
1968 LeMans conv. 350 - 4 speed Solar red/pearl
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