It's a big difference depending on how you got your 250 crowns. If you get one crown in each lesson and then moving on, then you probably have a rather vague grasp of a sizeable portion of the course. If, on the other hand, you take each skill to five crowns before moving on then you will be very sure of a much smaller portion of the language. These are very different levels of knowledge but both could be 250 crowns. Basically comparing is useless, both between crowns and the old system and from one crown user to another since you can use the system is so difference ways.
Do you mean the old Duolingo Fluency rating or how Fluent you are in German? Before crowns my Duolingo fluency varied between 68% and 69%. It stayed that way for a long time. I am now at 316 crowns, so I assume my old Duolingo fluency rating is about the same as it always was.
If you're asking about fluency in the language, I tend to rely on the CEFR model that recognizes six levels: A1, A2, B1, B2, C1 and C2, where C2 is considered highly fluent. Currently, I consider myself at an A2 level, pushing a pre-B1 for reading. Based on the CEFR model, I am at level 2 out of the 6 which would put me at 33% fluency.
Your actual mileage may vary.
I don't think it makes sense to draw a direct correlation between your crown level and your fluency.
OK, there are about 120 skills in the German tree, so if you get 5 crowns in all of them (thus completely gilding the German tree), you'd have a crown level of 600. If you have a crown level of 250, that's about 42% of 600, so you could say that you've completed 42% of the gilding. But does this actually correspond meaningfully to a fluency level? I don't really think so.
First of all, keep in mind that Duolingo itself is not meant to bring you to C2-level fluency. It doesn't even bring you to B2-level fluency; at best you might get somewhere around A2 or B1. So even if you fully gilded your tree in Duolingo, this doesn't mean you have native-level fluency in a language.
That said, I honestly think that anyone who has already gotten most of their skills up to crown level 3 has probably started to outgrow Duolingo. There isn't a whole lot of value in repeating the same exercise 30 times. You could do it for fun if you're a completionist who likes that kind of thing, but at some point you're just performing an act of repetition that isn't actually increasing your fluency in the language. So don't think that making the leap from, say, 300 crowns to 400 crowns is going to significantly boost your fluency.
Crowns and fluency do not equal, because fluency decreases when you do not streighten lessons. Crown levels stay forever no matter what you forget.
Duolingo is not the place to thoroughly and comprehensively practice and master a language. It will only help you cover some of the basics and give you a general overview of some grammar points, but if you want legit and solid progress you will have to learn grammar and vocabulary on your own, and most importantly communicate with natives as much as possible. Maxing out each skills won't bring you much, especially since exercices are insanely repetitive and do not force you to make sentences of you own, once you've learned a reply you're done, and past the 3rd crown, you've already learned everything, and none of the content gets any harder. I suggest using duolingo as a side help and do some real learning as your main thing.
You'll be very good at constructing and hopefully listening to simple sentences with a vocab of about 2500 words.
So not very far considering that to be fluent, you need something like 10K words (bare minimum) and the ability to (easily) speak (not just write) complex sentences.
Basically, DuoLingo is a great first step, but you'll need to start branching out soon after: reading books, listening to media, finding larger vocab sources, and most importantly, conversing with native speakers.