Theretically speaking, the average net earning rate is extremely hard to get.
It is more likely than not to be a conceptual number.

Think about the earning formular, [price - COGS].
For the all known reason, the COGS is not accurate at all, not to mention the volatile price tendency for the promotion reason. My conconlusion is, it is really impossible to get a relaible average net earning rate.

可以用GDP增长率替代吗？
Absolutely not.
They have totally different economic and accounting meanings.
A nation could generate a relatively high GDP growth rate, but actually has a moderate average earning rate.

Another evidence is, our recent GDP growth rate is about 5~7.5%.
Does that mean that our average social earning rate is that low?
No way, it should be at a higher level,  right?

The last but not least, entity will invest part of its earnings in R&D,
citizens will consume much of its wealth on a daily basis,
Those negative impact definitely leads net earning rate far away from GDP growth rate.

You may derive your index from national statistic website.
Or ask realy expert for an academic guess.
Above all, use your reasonble doubt to judge the number please.

I hope this helps.
Thanks.