Here's the spreadsheet:
https://docs.google.com/spreadsheet/...WFBaTJZZk1BVVE
You have to make a copy or download it to use it.
The yellow fields are the ones you should be entering numbers into. Don't touch the others.
In the top left, just enter the numbers that appear on your character sheet.
The field that says "change value" corresponds to the numbers below it. It is the amount of each stat that you'd be adding in order to gain the effective HP found in the fields E13-17.
So for example, the spreadsheet has my current stats listed with a change value of 10. This means that an increase of 10 vitality will increase my effective hp by 0.78%, but an increase of 10 resist all will increase my effective hp by 1.75%. I made it so that armor changes are multiplied by 10 to make it more of an "apples to apples" comparison between the stats. The Life % change number is not effected by the change value; I made it a constant 1%.
The numbers in the F column are the ratios of each E column value with respect to vitality. This is just there to make it easier to see how much each stat is worth in terms of vitality. For example, right now the F column indicates to me that 10 more resist all is worth the same as 22.5 more vitality.
I think the I and J columns are pretty self explanatory. Enter the values from two pieces of gear in, and the L11 cell will tell you how much your effective HP will increase or decrease as a result.