I have a set of two variables log-normal distributed, called P99 and
P93. They define the equity of many companies which could be negative.
In order to fit the relationship between both I made a logarithm
tranformation:
P99->L(P99+C)
P93->L(P93+C')
Where C and C' are the minimum values of the observation (remember,
they´re negatives).

The question is: may C and C' be both minimums for both variables or
may C=C' and equal to the minimum value of P99 and P93? The answer
must be made in order to make a right regression in the form
L(P99)=aL(P93)+b which right coefficient "a"