Using one specific value from logaritmic trendline

[Remco]

Hello again to all,

I have an algorithmic trendline calculated including future trend but now I want specific year to be used as index to 100% and than calculate the percentages for the coming years. The formula for the algorithm is:

round(linest_m(total aggr(round(sum(GROUPPERFORMANCE) / sum(UNITS),0.001),STEP),log(STEP)),0.0001)
* log(STEP)
+ round(linest_b(total aggr(round(sum(GROUPPERFORMANCE) / sum(UNITS),0.001),STEP),log(STEP)),0.0001)

and beside other values, it gives my table. But I want ta add a column where 2020 is set to 100% and the next years should be calculated. 

So, 2020 becomes 100% and 2021 would be 50,1 / 49,8 * 100 = 100,6%

But how can I get this fixed calculated value for 2020 to be used in the other years?

The table:

 

Thanks!