The topping lift is normally one of two varieties. One is a static line, secured at or near the top of the mast, brought down to the boom, and secured to some attachment point there. It has to be long enough that the sail will fully support the boom when raised - the topping lift must be slack under all conditions - unless you want it not slack, for sail shape reasons, but that's taking it up a notch. The other way is to pass the topping lift over a small block and bring it down the mast, so it can be adjusted from there, which is how mine is.
It really doesn't need to be adjustable from the mast if you make it long enough to make it adjustable at the boom end, though. Just that much more clutter at the cleats. In fact, I may just replace it with a fixed length, now that I've said it out loud and given it a little thought.
But I set mine at the mast to a specific length (I put a black mark on it for reference), long enough to never be under tension when the main sail is raised, and from the cockpit I adjust the TL by winding it around the boom to raise it high over my head once the sail is flaked and covered.
If I'm leaving the boat unattended for long periods of time, I move the main sheet to a fitting on one of the gunwales and use a small line with snap hooks at each end to a matching fitting on the other gunwale so the boom is held centered on the boat and can't rock as the boat moves. Looks like an upside down wye. But if I'm using the full bimini/bridge/dodger compliment, I can't do that (canvas is in the way), so I just run the main sheet to the life line loop on the rail on either side and tension it so it doesn't move around too much. Again, the topping lift holds it up, which is what it's there for.
Oh, and there are other things folks do to replace the TL, like a boom kicker or rigid vang. But since you have the TL, there's no reason to think beyond it until you've spent some quality time on the water with her.
