You only do the overnight FC loss test when you suspect a problem, such as losing chlorine faster than normal for your pool.

To switch, you just start using chlorinating liquid or bleach as your primary source of chlorine. However, with the Sustain Shield chemical in the pool, it's unclear how to deal with that and transition to using CYA. The only clear-cut way is a full water replacement, but perhaps you can transition over time. If you see the chlorine loss increasing during the day but not at night, due to the Sustain Shield slowly getting diluted out of the pool, then you can increase your Cyanuric Acid (CYA) level. Unfortunately such an intermediate combination isn't something we know anything about nor have documented for what to do.

As for how much chlorine to add, you add whatever is needed to not go below the minimum in The PoolSolutions Best Guess Swimming Pool Chlorine Chart, but that would be after you get rid of the Sustain Shield from your pool. As purely a guess, I would follow this chart when the CYA level gets higher, but when it's below 30 ppm CYA as it is right now I'd still have a minimum of 3 ppm FC at all times. As Ben mentioned, you can use Polyquat 60 weekly as insurance to prevent algae during this transition period until the Summer Shield goes away.