Do I have to use the 3.5 gpf for baseline and installed blow out fixture if installed fixture is lower than 1.6 gpf. (3.5 gpf seems to be a mandatory requirement if fixture, tipically blow out fixture, is higher than 1.6gpf).

If so can I use 1.6gpf for baseline case and the installed value (lower than 1.6 gpf) for installed case?.

If not, if we had to employ 3.5gpf both for baseline and for insalled case, it is a very hard (nearly imposible without using nonpotable water reuse) task to comply with 20 % savings due to the higher amount of water required for flush fixture in relation to flow fixture water reqeuirtement ( up to 10/1th ) and even imposible if building roof area is not big enough to collect rain for nonpotable uses.
And as 20% is a Prerequisite it would be not posible to certify a small roof surface building if blow out fixture are used.
Here in Spain Blow out fixture can have tipical values of 1.6-2.3 gpf and smaller and are required by local rules when it is required to reduce water use in public WC instead of water closet WC!!