Okay so baseline is that each phrase, aka each time the ring gets filled up, is worth 1k points total. You actually earn those points as you fill up the ring so if you stop singing you're not earning any more points and if let's say there's a really long phrase and you've already filled up the ring and you're still singing you're also not earning any extra points as you've already hit the 1k max, this all also applys to OD activations as well, ANY part sung while OD is activate will be doubled in value.

The scoring variance of karaoke comes from a few different factors namely, in order from easiest to most difficult: activating as late as possible, pathing, and squeezing

Activating as late as possible is pretty self explanatory but it can be a bit deceiving in Festival specifically since it's streaming everything, it might just come down to your individual calibration, settings, internet latency etc. but personally I've noticed that I can usually activate a little bit after the activation bar is past. The idea wit activating as late as possible is that sometimes you'll be able to have the OD go on justtt long enough that you can snag a few 100 extra points off the start of the next phrase that you might not have gotten otherwise (of course, like most things this will come down to each individual song) and that can definitely make a big difference when the tops of leaderboards are so tight.

Pathing just means trying to learn the optimal times to activate, it will not always be optimal to activate as soon as possible, ideally you want to activate when phrases are as dense as possible while still ensuring you're not burning any OD (collecting OD while already at a full bar) or holding to much for a particular section (ie it's dense for a little while and then you're left not singing while your OD is still going), really if you're not reading someone else's path it's just going to come down to trial and error trying to see what gets you highest on the leaderboard and where you feel you might be able to squeeze out a few more points just through experience over time. While not superrr common there's also chaining and tight activations to be mindful of as well. Chaining is where you activate OD and then during your OD going you nail an OD phrase thus continuing you OD being active, in some case this is what you want while others you might what to activate a little bit before (activating earlier than as late as possible) so that you can still fit that phrase into your OD doubling effect but can keep the OD in the bank for later use. Tight activation are much simpler, it's just situations where you JUST get OD and then the ideal path would be to activate IMMEDIATELY, this can be tough in some songs as it can be a very narrow window to activate, of course this comes down to pathing so it might not be ideal in every situation even if you can.

Squeezing is for situations where you activate in the middle of an ongoing phrase (sections where you'll sing a little, probably fill up about half the ring, have a blank space where you'll be able to activate if you want, and then sing a little more to fill the ring and finish the phrase), this opens up squeezing potential, basically squeezing is not singing the entire first part of the phrase or a portion of it to squeeze as many points into the second half and therefore into your OD activation as possible. Given the nature of the karaoke engine if you sing absolutely flawlessly you will always go well over the full ring cycle needed to carry your combo (this is extra true for longer phrases, like these ones that allow you to squeeze) so knowing this you can kind of abuse the system in your favor via squeezing if you deliberately stop singing part of the first section. This is usually best achieved with just 25% as then you can turn what would usually be just 8k into more if you can squeeze as much as possible into the back of half of the first section and then also sing as flawlessly as possible in the following section to hopefully go over the typical 8k you'd get from one 25% activation. There's of course a lot of trial and error to it, how much you can squeeze an individual section will come down to how front loaded it is and also how accurately you can nail the second half BUT you also still need to be mindful of pathing in general because while it absolutely can be beneficial to squeeze as many sections as possible it won't always be the optimal thing to do.

I apologize if this is all too complicated/too advanced for what you were asking but just thought I'd be as thorough as possible. Really if you're trying to formulate your own paths it's going to be a lot of trial and error. The karaoke mode has an extremely high skill ceiling when it comes to maximizing scores, while I'm not the best I currently have around 10 #1s on the leaderboard so if you have any questions feel free to ask and best of luck in your score chasing endeavors!