Actually, that's not quite how it works.
In order to opt in to ad revenue or tipping, you need 100 or 250 subscribers on a single series to unlock that feature for all (at least, I think all) your series. In your example, you would only have 70 subscribers counting towards the ad revenue milestone.
I currently have 92 subscribers in total; however, only 58 subscribers actually count towards the ad revenue milestone because that's how many subscribers are on my largest series.