"Value" is subjective. It is only defineable by each individual and their assessment of the match up and the prices involved. Ultimately, any pick anyone makes is by definition seen as value by them otherwise there would be no point in making it.
If you like the match up and you are happy with the price, then to you the pick is value and worth making.
With regards to how much is being staked - the best way to do it is minimal stakes. However, as you get older and more money is available then it's always likely that the stakes will increase. That inturn can give you bigger wins and likewise bigger problems. It's a personal decision and when I started when I was 14 I never imagined I'd stake as much as I do now, but that's the way it goes. As it happens, my really serious days of gambling are actually behind me, even though my stakes are higher than the average punter, but it's all about what you can afford to play with, what you are willing to risk/lose should the worst come to the worst.
I've never subscribed to thinking that you should "only back favs" or "only back dogs". That's just stupid. It's not about that. It's about assessing the match ups and evaluating the prices. Sometimes it'll be favs and sometimes it'll be dogs.
Any pick anyone makes should always be evidenced by something factual or tangible to make the pick viable. So yes, backing Levine against Nadal even at astronomical odds does seem futile to me, because I can't make a case to myself as to why Levine should have the necessary to win. And even if you make the pick with the vision to trade, at a break up or even a set up, Nadal's price is never going to be backable at that stage to make it worth off setting.
However, there are matches where the trade value is there, but again that's just a judgement call for each individual.