It sounds like a sensible amount to begin with, except I would do it every 3 hours instead of 4. You could probably double that amount next round , do that a couple times, then do 50mg every 3 hours for a few rounds before adding in ALA.
It is also possible that most of your mercury storage is inside your cells/organs/brain, in which case you may not feel effects until taking ALA. If you have done a lot of garlic in the past this may have minimized your surface level mercury. Either way, it is good to continue diligently with the DMSA since you will be lowering lead and other metals too.