for example on a viper

I know max def is 1034, but where do I check the defense range it could have when upped?

So an upgraded vipermagi's defense will vary from 364*2.2 to 470*2.2, that is 800 to 1034.

Some of these things you'd just have to search online for. Other option is posting in the d2r discussion topic and asking. The link for that spot is below.

https://forums.d2jsp.org/forum.php?f=148

A week old post but you can find alot of them on my profile https://forums.d2jsp.org/user.php?i=199374

Its pretty easy to do the math

For vipermagi for example, you have yo use a white Wyrmhide def (because it is the elite version on serpentskin armor) It varies from 364-470 and you add 120%. So, once upped def will vary from 364+120%=800,8 (801 rounded up) to 470+120%=1034

You can find whites exact stats on arreat summit or on maxroll

Dont forget, d2r always rounds down

800 is lowest defense