How isotopes atomic mass was defined? So we say that 1 Amu (u) = 1/12 of the mass of a Carbon 12 atom.

Im not sure about this, but I believe that first we measured  the mass of 1 carbon 12 atom (This includes the mass defects that occur during the formation of nucleus) and then defined that 1 atomic mass unit has a value of 1/12 of the carbon 12 atom mass... Therefore one Carbon 12 atom has exactly 12 atomic mass units.

Until here everything is fine.

Now my question is... To define all the other elements isotopes atomic mass did we first measured each isotope atom mass and then said " If 1/12 of the Carbon 12 atom is equal to X grams and this X grams are equal to 1 atomic unit the then my isotope atom which has Y grams must have Y atomic mass units " did we work it out this way or did we use calculation to define other isotopes mass based on carbon 12 atomic mass ?

Basically what I want to know is, do we need to have the value of an atoms mass in grams or kilograms before we know how many Amu´s it has, or can we calculate it ?
Historically, the relative masses of the isotopes were measured relative to O=16.  

There is a good historical explanation in A New Mass-Spectrograph and the Whole Number Rule (1927). 

At this time (1927), though it was understood that many elements had more than one isotope, only one isotope of oxygen was known.