Let’s say there’s 10 billion people.
504,921,600 grin / 10 billion =
0.050492160 grin per person
50,492,160 nanogrin per person?
—-
For bitcoin it’s 21,000,000 / 10 billion =
0.00210000 bitcoin per person
210,000 sats per person?
I am pretty sure everyone in bitcoin land is counting on using some made up extra digits as part of a layer 2 in order to be usable.
Right on the mark 
. On the Lightening network payments are defined in millisatoshis, so in theory it is already possible to make sub satoshi transactions. Cool thing is that in that way you can already do streaming money payments on Lightening. I do not know if it is in practice allowed by most lightening nodes. Probably not to avoid spamming with micropayments
And here I was aiming to accumulate 1 grin day (86400ツ) for each of my kids. An ambitious goal that I am far from achieving, yet I slowly accumulate for them.