DataKid, dont try to squeeze the numbers to find a reason why you would be "allowed" to play at NL25.
1/ You can of course "take a shot" with 400$ at NL25. Nothing bad with that, but you need to be able to move down if things go wrong.
2/ The 20 buy in rule, is not really changed if you decide to play YOURSELF with a 40bb stack instead of the "traditionnal" 100bb buy in. Because when you think of it, the pots at the table will be of the same size, the blinds are still the same 0.10/0.25 wether YOU play with 40bb or not.
Because if we push this reasonning to the extreme, then why not play NL100, say, if you buy in only for 10$. But this is of course not true.
The 20 buy in rule is based on MAX buy ins.
Take shots maybe, but be aware this is shots we are talking about.