This spreadsheet is composed by two parts:
Power, Precision and Critical Damage Balance (left)
Toughness and Vitality Balance (right)
The purpose of these two models is to optimize a character so he can unleash the maximum damage possible combined with the maximum possible survivability. Of course, having the perfect balance is nearly impossibile. However you can get pretty close to it using the science i'm giving you.
Power, Precision and Critical Damage Balance
You need to insert your base Power, your critical chance in decimals (not 49% but 0,49) and your critical damage also in decimals (not 209% but 2,09).
After that, select if you want to use the WvW guard stacks and Bloodlust/Perception stacks and, for the might stacks, i always recomand that you use 10 as value (usually everyone in a decent group has 10 might stacks in combat!).
The spreadsheet will now tell you if you are unbalanced or mostly balanced toward power or precision, so you may do the necessary corrections to your build.
Toughness and Vitality Balance
As before, you need to insert some values:
HP Unbuffed: your standard health points.
Heal 1,2,3 and 4: all the health points that you can heal yourself before dying A.K.A the sum of all your heals.
Armor: your "Armor" attribute.
Coefficient: this is a subjective measure that represents the percentage of direct damage you normally take (A.K.A. non-condition damage). The higher the coefficient is the less vitality you would probably need. A examples for setting the right measures are:
PvE: 0,9 (usually you don't take much condition damage in PvE)
WvW: 0,75 (0,8 / 0,85 if you have the -40% condition duration food or good condition removals)
Also remember that the more condition removal skills you have the less you are likely to take condition damage.
Once you're set the model answer the question: do i need toughness?
The answer lies in the number under false/true: 10 is the perfect balance, if lower you need vitality, if above, you need toughness. Of course, the spreadsheet will tell you directly what you need to raise.
I also added a survivability index to measure your "tankiness":
3501 - 4000 - Fragile
4001 - 4500 - Average
4501 - 5000 - Durable
5001 - 5500 - Tanky
5501 - 6000 - Build to last
6001 - Moving fortress
To finish i added a graph showing how your damage reduction raises with the armor attribute. As you may notice, once you go past 3000 armor there are heavy diminishing returns. I advise to never go above that number.