Thanks for the feedback/update. I've just had a quick look, and I can't find anything on the social security website about €70 etc, but perhaps I didn't look far enough. All I found was:
This says at section D.2. MODIFICACIÓN DE LA DENOMINADA “TARIFA PLANA” that the starting rate has gone up to €60.
Looking at what appears to be the relevant law:
https://www.boe.es/buscar/doc.php?id=BOE-A-2018-17992
This says:
In the event that the minimum contribution base has been chosen, the contribution during the first 12 months immediately following the date of effects of the registration will consist, as of January 1, 2019, of a single monthly installment of 60 euros.
The only conclusion I can draw is that the current minimum rate for 2019 is €60, but 'In the event that the minimum contribution base has been chosen' implies that there must be other rates for different grades of autónomo.
Sorry, but it looks like you'll have to take the advice of the experts.