reescaling ?

Hello to all.

I'm analizyng a data composed by several paired observations. The first observation is non treated that is the baseline of signal. Then we record the same after several days of treatment. This is done for 5 different groups with 4 repicates. The problem is that the baseline is different for each observation, so i consider to reescale to maximun and minimun each observation to compare. Other possibility is calculate the ratio betwen first and second observation. Wich coul be the best approach? and wich test is more convenient to compare groups.

Thanks in advance