If you have 'made-up' the level at all. - added material. Then no matter how well you firm it, it will settle.. You either have to make it slightly high, and wait for it to settle. Or, accept there will be subsidence, and then correct as necessary.
Not an exact science I am afraid. Even full-size railways have to tweak things.. The problem for us, is an inch of settlement, in real measure, is the best part of two feet at scale.
The more time spent getting it right during construction, the better it will last, and the less adjustment later. BUT, it will need attention occasionally.
You are 'running a railway', not just operating the trains! You have to do it all. - All part of the fun, honest.