In case macrid ever comes back and is interested or something
I couldn't find it anywhere, so I worked out that bloodthirsters shield amount is:
((Level - 4) * 30) + 20
Actually for that to work the minimum of 50 thing must be a base value. Level 5 onwards and this works out.
I'm sure someone else has found this one out but it's not on the wiki or anything
Also here are some numbers about infinity edge against bloodthirster:
'oh but bloodthirster will beat infinity edge at high AD values right?'
No, Infinity Edge will always win apparently. That said you really do need lifesteal in your build.