Please find the article from Memory Alpha on NCC-numbers below:
http://memory-alpha.wikia.com/wiki/NCC
Some of the lowest numbers, like NCC-42 and NCC-176 were found in TNG era, therefore implying Discovery will be set between ENT and TOS basing only on the number being lower then Enterprise's is definitely wrong. These numbers look like a mess to me, there is also a possibility that after a ship is destroyed/missing/whatever, a new one can be assigned to that number - just like we had different Enterprises with the same number and only a letter added.
http://memory-alpha.wikia.com/wiki/NCC
Some of the lowest numbers, like NCC-42 and NCC-176 were found in TNG era, therefore implying Discovery will be set between ENT and TOS basing only on the number being lower then Enterprise's is definitely wrong. These numbers look like a mess to me, there is also a possibility that after a ship is destroyed/missing/whatever, a new one can be assigned to that number - just like we had different Enterprises with the same number and only a letter added.