Yes, he is on borrowed time. But you are now on a plan to make him healthy. The 6 dose plan developed on this FDMB site is to keep kittehs safe as their dosage is increased.
One of the bigger issues with unregulated kittehs is hind-leg neuropathy. It doesn't always happen. Leo had it, and it is so devastating...we almost made "the decision" before he made the slow road to recovery. That was really a nightmare for us, and Leo.
One thing you could do is give Tucker some daily Zobaline B12. It helps the neural sheaths by replenishing the B12 in them. Once Tucker is regulated, this should not be a concern.
I do not think this dosage level will be sufficient. But keep playing it safe with the 6 dose protocol. Many owners target nadirs of 80-120, so Tucker is still a long ways from achieving that. But he's getting there.