I have virtually no knowledge when it comes to scanning but if scanning is your workflow you could still apply some basic exposure/development testing. The idea of establishing your own working exposure index (ie film speed) is to make sure you give enough exposure to get the shadow detail you want in the negative. As others have said this is to some degree a matter of personal preference, how you develop your film, how you print/scan etc.

With a general purpose developer like HC-110 and normal development/contrast most peoples' speed ratings with a film like Acros will end up somewhere between 50 and 100, typically closer to the 50 end. Actually with most films many people prefer a similar approach (ie rating the film anywhere from a half to one stop lower than box speed). But again you'd have to decide for yourself. Generally, Acros benefits from a little more exposure and a little less development since it inherently has very high highlight contrast.