Given that the 90 is already bent, the same calculations as for an offset will give a pretty close answer. Referring to the diagrams in the article, the length of C will be the length of A divided by the sine of angle D. Measurements will be taken from the back of the 90. The figures will not be exact because of the difference in pipe thickness but will be quite close
Shrinkage can be calculated as C - B, with B = cosine d * C.
But that is all figured with the 90 already bent. I suspect you are asking where to put that 90 as well, and that becomes a very different proposition as different pipes will have a different radius of bend. The best you could realistically do is figure where the 90 should be bent as if there would be no kick, then add the shrinkage value to that number. Once bent, treat it as if it were an offset using the same multipliers as you would for bending an offset. Again, it will not be exact, but might be close enough for practical application in the field.