As a general suggestion, name your cells!!

I played arround with your formula :-

(I colorcoded your 2 constants below, not sure if they will always be the same) if so it could possibly be simplified further!

=ROUND((((GrossRealisation-(LegalPS+(SComissPC*GrossRealisation)+(MarketingPC*GrossRealisation)+(GrossRealisation-(GrossRealisation/1.125))))-(((GrossRealisation-(LegalPS+(SComissPC*GrossRealisation)+(MarketingPC*GrossRealisation)+(GrossRealisation-(GrossRealisation/1.125))))*(ProfitRiskPC/(1+ProfitRiskPC))))-(DirectCosts+HoldingCosts))*1.125),-3)
It makes it more readable to change!

By naming the variables across the side and top eg k10:q10 as GrossRealisation you can use it in your formula! And still copy between the cells, but its easier to maintain!

also a little math and simplification :-

=ROUND( (((100%-($C$11+$C$12+(1-(1/1.125))))*K$10-$D$10)/(1+$C$15)-(K$11+$J12))*1.125,-3)
With Names:-
=ROUND( (((100%-(SComissPC+MarketingPC+(1-(1/1.125))))*GrossRealisation-LegalPS)/(1+ProfitRiskPC)-(HoldingCosts+DirectCosts))*1.125,-3)