jlo212 - these 3 ISX ASX statements below (and/or attached)...

jlo212 - these 3 ISX ASX statements below (and/or attached) would need to be factored into the equation/analysis of what you have requested above from 'others'.  

The ISX merchant base is (i) constantly and quickly evolving, (ii) being added to (types of merchants and/or processing types/volumes) and as such (iii) volume/$ processing is rapidly changing especially if you compare late 2018 announcement and early 2019.

Factors which will impact and escalate new application requests and processing will be, in part and for example,  2019 Brexit, EU compliance policies from various jurisdictions ('reverse' Australia and overseas) - and also the Australian Royal Commission bank outcome for the need to KYC - plus the need for the client and their bank or financial organisation to have more rapid/speedy KYC authentication.  These are only a few of the most recently (non-ASX publicised) external factors which will have merchants seeking the services of ISX or similar.  I don't know of anyone outside of ISX who is offering this service to merchants nationally or internationally.

I read from recent ISX announcements that the merchant organisations' needs are changing and that the merchants have differing and different processing needs.  Plus the merchants operate in many countries within different jurisdictions.  

I think it unlikely anyone could accurately guess the "maths" you are seeking given the variables listed above/and attached.